|Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact|
|Full name||PDF (Portable Document Format) Family|
PDF (Portable Document Format), developed by Adobe Systems Incorporated, is described by Adobe as a general document representation language. PDF represents formatted, page-oriented documents. These documents may be structured or simple. They may contain text, images, graphics, and other multimedia content, such as video and audio. There is support for annotations, metadata, hypertext links, and bookmarks. Later versions provide additional functionalities, for example, to embed geospatial information within documents that represent maps or other geospatial images, such as satellite photographs.
At the core of PDF is an imaging model derived from the PostScript page description language. See Postscript Format Family. This model enables the description of text and graphics in a device-independent and resolution-independent manner at a complete, precise and professional level. Unlike PostScript, which is a programming language, PDF is based on a structured binary file format that is optimised for high performance in interactive viewing.
|Production phase||In general, a final-state format for delivery to end users.|
|Relationship to other formats|
|Has subtype||PDF_1_3, PDF Versions 1.0-1.3|
|Has subtype||PDF_1_4, PDF Version 1.4|
|Has subtype||PDF_1_5, PDF, Version 1.5|
|Has subtype||PDF_1_6, PDF, Version 1.6|
|Has subtype||PDF_1_7, PDF, Version 1.7 (ISO 32000-1:2008)|
|Has subtype||PDF_1_7_ext03, PDF, Version 1.7, ExtensionLevel 3|
|Has subtype||PDF_1_7_ext05, PDF, Version 1.7, ExtensionLevel 5|
|Has subtype||PDF_2_0, PDF 2.0, ISO 32000-2. PDF, Version 2.0 (ISO 32000-2:2017).|
|Has subtype||PDF/A_family, PDF for Long-term Preservation. As of November 2012, there are three chronological versions of PDF/A.|
|Has subtype||PDF/A-1, PDF for Long-term Preservation, Use of PDF 1.4|
|Has subtype||PDF/A-2, PDF/A-2 for Long-term Preservation, Use of ISO 32000-1 (PDF 1.7)|
|Has subtype||PDF/A-3, PDF/A-3 for Long-term Preservation, Use of ISO 32000-1 (PDF 1.7), with Embedded Files|
|Has subtype||PDF/E-1, PDF Engineering Document Format, Use of PDF 1.6|
|Has subtype||PDF/UA-1, PDF/UA-1, PDF Enhancement for Accessibility, Use of ISO 32000-1|
|Has subtype||PDF/X, PDF for Prepress Graphics File Exchange|
|May contain||PDF_geospatial, PDF, Geospatial encoding (Adobe). Supported by version 1.7 ExtensionLevel 3.|
|May contain||GeoPDF_2_2, GeoPDF encoding (TerraGo), version 2.2|
|LC experience or existing holdings||Used as service format, including for some scanned historical materials, primarily to support convenient downloading and printing. Acceptable format for copyright registration.|
The Library of Congress expresses preferences for formats for content for its collections through through two venues:
Fully documented. Many members of the PDF family of formats were developed by Adobe Systems Incorporated, which made the specifications available openly and at no charge. Several members of the family have been adopted as ISO international standards, e.g,. PDF/X (ISO 15930), PDF/A (ISO 19005), and PDF version 1.7 (ISO 32000-1:2008). These standards are available for sale, primarily through national standards bodies and approved agents. Additional information about specifications and standardization is provided in the format descriptions for several of the subtypes.
Since the approval by ISO of PDF 1.7 as ISO 32000-1:2008, maintenance of almost all PDF specification has been under the auspices of working groups of ISO TC 171 SC 2, with the main PDF specification under WG8. From 2002 to 2016, AIIM (The Association for Information and Image Management) acted as secretariat and U.S. Technical Advisory Group (TAG) to ISO/TC 171 SC 2 (see AIIM | U.S. TAG to ISO/TC 171 from 2015). In 2017, the 3D PDF Consortium was approved by the American National Standards Institute (ANSI) as a standards developer and has assumed the role of secretariat and U.S. TAG Administrator for ISO/TC 171 SC 2 (see 3D PDF Consortium Approved by ANSI as US TAG Administrator for PDF ISO Standards).
See Specifications below for links to the ISO pages for information about ISO 32000-1:2008 (PDF 1.7) and ISO 32000-2:2017 (PDF 2.0).
Adobe provides free access to documentation for PDF 1.7 at http://www.adobe.com/devnet/pdf/pdf_reference.html and an archive of earlier versions at http://www.adobe.com/devnet/pdf/pdf_reference_archive.html.
Extremely widely adopted as a platform-independent format for disseminating page-oriented documents. For some general observations on trends, see the slides in PDF statistics – the universe of electronic documents, from a talk by Duff Johnson at PDF Europe Days in May 2018.
One area in which PDF is particularly widely used is for prepress workflows. See How PDF changed prepress production dramatically in the last 25 years, a blog post from October 2018.
Adobe Reader software for viewing PDF files is freely distributed and bundled with most personal computers.
|Licensing and patents||
Adobe has a number of patents covering technology that is disclosed in the Portable Document Format (PDF) Specification, version 1.3 and later.
An annotated summary of information on the Adobe Web site in September 2010 (see http://partners.adobe.com/public/developer/support/topic_legal_notices.html via Internet Archive) follows. Note that, based on a 20-year period for U.S. Patents, all the patents listed on this Adobe page are expected to have expired in the U.S. by 2019-05-06. Comments welcome.
To promote the use of PDF for information interchange the following patents are licensed by Adobe on a royalty-free, non-exclusive basis for the term of each patent for developing software that produces, consumes, and interprets PDF files : 5,634,064 (filed 1996-08-02, granted 1997-05-27, probably expired as of 2019-03-01); 5,737,599 (filed 1995-12-07, granted 1998-04-07, probably expired as of 2019-03-01); 5,781,785 (filed 1995-09-26, granted 1998-07-14, probably expired as of 2019-03-01); 5,819,301 (filed 1997-09-09, granted 1998-10-06, probably expired as of 2019-03-01); 6,028,583 (filed 1998-01-16, granted 2002-02-22, probably expired as of 2019-03-01); 6,289,364 (filed 1997-12-22, granted 2001-09-11, probably expired as of 2019-03-01); 6,421,460 (filed 1999-05-06, granted 2002-07-16, probably expiring 2019-05-06). Patent 5,860,074 (filed 1997-08-14, granted 1999-01-12, probably expired as of 2019-03-01) is similarly licensed on a royalty-free, non-exclusive basis for its term but only for the purpose of developing software that produces PDF files (thus specifically excluding software that consumes and/or interprets PDF files).
A similar statement was submitted in 2004 to IETF as part of Adobe Systems Incorporated's Statement about IPR claimed in draft-zilles-pdf.
In association with the adoption of PDF, version 1.7 as an ISO standard (ISO 32000-1:2008), Adobe issued a Public Patent License, granting "every individual and organization in the world the royalty-free right, under all Essential Claims that Adobe owns, to make, have made, use, sell, import and distribute Compliant Implementations."
|Transparency||Depends upon compliant software tools to read. Building tools requires sophistication. In most PDF files, the content is compressed. Many PDFs include embedded content in binary form, for example for images, and for annotations in audio or video. See Notes below for more detail on compression of "stream objects" in PDF files.|
Starting with PDF 1.4, chronological versions of PDF can include metadata "streams" in the XMP (Extensible Metadata Platform) format at the level of the document or for individual objects. The XMP format, developed by Adobe in 2001, is a framework for including arbitrary blocks of metadata, using a representation in RDF. XMP was approved as ISO 16684-1:2012 in 2012.
Use of document-level XMP is mandatory in some standard subsets for PDF, including the PDF/A_family.
|External dependencies||Faithful rendering requires that fonts be embedded. PDF variants in the PDF/A_family, intended for archival purposes, PDF/X, for prepress exchange, and PDF/E-1, for engineering documentation, require that fonts be embedded.|
|Technical protection considerations||The PDF format offers several forms of technical protection, including encryption and password protection, that would prevent custodians of digital content ensuring accessibility in future technological environments.|
|Normal rendering||PDF is designed for page-oriented documents. Scaling, zooming, printing are expected functionalities for PDF viewers. The quality of raster images depend on the quality of the embedded image. Note that, in general, PDF is not a preferred archival or master format for images.|
|Clarity (high image resolution)||High-resolution images can be embedded using professional tools. See PDF/X, a standard version of PDF used by the printing industry.|
|Color maintenance||Parameters to support color management, including CIE-based and ICC-based color spaces, can be stored in the file using professional tools. See PDF/X, a standard version of PDF used by the printing industry.|
|Support for vector graphics, including graphic effects and typography||Extensive support for graphic elements. Versions after PDF 1.4 support a transparent imaging model in addition to the opaque model used for earlier versions. Hence images composed of layers can be stored without pre-composing into a single image.|
|Support for multispectral bands||PDF is designed to support printing and visual display on screens. PDF is not designed to support analysis of multispectral image data, which can include bands/channels outside the visible range. For scientific communication, raw multispectral image data may be converted to an image in a color space and format supported in the PDF specifications. See, for example, Introduction to Image Processing from the website of the Hubble space telescope. Comments welcome.|
|Functionality beyond normal rendering||PDF has extensive support for annotations of several types. PDF, Version 1.7, ExtensionLevel 3 (PDF_1_7_ext03), introduced with Acrobat 9.0, supports capabilities for embedding data in association with points within 3D and geospatial images.|
Good support is possible, but not guaranteed. The PDF format allow creators to disallow printing and extraction of text for quotations. PDF can also be used to create documents from scanned page images; such files do not necessarily support indexing of the document text.
Although for most PDFs that do incorporate character-based text, the text can be reliably extracted and indexed, problems can occur, because the PDF internal structure for text is based primarily on identification of glyphs within fonts and not on Unicode code points. If Unicode code points are not present, perhaps in order to make the file as small as possible, extracted text will be unintelligible. See Why is the extraction of text from a PDF document such a hassle?, a blog post by Dr. Hans Bärfuss of pdf-tools.com. See also a useful reponse to the problem Cannot copy non-latin characters from PDF document on Stack Exchange.
|Integrity of document structure||The logical structure of a document is only represented in a PDF file if the creator or process during creation takes steps to incorporate structural tagging.|
|Integrity of layout and display||PDF is designed to represent the layout of page-oriented documents.|
|Support for mathematics, formulae, etc.||
Can be represented visually by embedded graphics or using mathematical fonts and specialized tools for mathematical layout. See, for example, Sample of MathML 2, a PDF 1.4 document from Antenna House, a vendor of specialized typesetting tools. Antenna House Formatter can use XSL-FO (XSL Formatting Objects) to format MathML for printing or display. The typesetting system LaTeX has a Math Mode that can be used to create PDFs. See Very Basic Mathematical Latex, a PDF created using LaTeX that shows sample code and rendered equations and formulas.
PDF 2.0 introduced a standard way to incorporate the source MathML markup in a way that supports content re-use and accessibility via tools that convert MathML to braille. See, for example, Positive Impacts of EPUB 3: MathML and Braille Mathematics. A widely used software library for generating braille, Liblouis, has a version that can translate MathML into a braille representation; see Liblouisutdml User's and Programmer's Manual.
|Functionality beyond normal rendering||
Supports annotations and bookmarks.
Supports embedding of media objects (in binary format) and links to external media objects, such as images, audio, or video. Audio and video are considered "annotations." PDF Support for 3D artwork in the U3D format was added in Adobe's PDF 1.7, ExtensionLevel 3 and has been incorporated into PDF 2.0. PDF 2.0 also added support for 3D artwork in the PRC format.
|Internet Media Type||application/pdf
||Registered with IANA (see Application Media-Types), described originally in IETF (Internet Engineering Task Force) RFC 3778, which was obsoleted by RFC 8118. Reported for PDF files by JHOVE PDF-hul module for file identification.|
|Internet Media Type||application/x-pdf
|Selected media types listed at The File Extension Source.|
|Magic numbers||Hex: 25 50 44 46
|From Gary Kessler's File Signatures Table.|
|Indicator for profile, level, version, etc.||See note.||
PDF files should have a chronological version identified in the header with the 5 characters %PDF– followed by a version number. For example, PDF 1.7 would be identified as %PDF-1.7. However, this version identification can be over-ridden by a version value stored in the document's Catalog. See Notes below, for more detail.
|Pronom PUID||See note
||Pronom PUIDs at subtype level only.|
|Wikidata Title ID||Q42332
Maximum size for PDFs: This topic has been discussed in a number of online forums. At one Adobe forum https://forums.adobe.com/thread/1041350 (consulted in September 2012), a very high theoretical page-count limit is described: "There's no explicit page number limit but there is a limit on indirect objects of 8,388,607 in a 32-bit PDF rendering application--Acrobat and Adobe Reader are both 32-bit code--and because each page consumes at least one indirect object, every PDF file created by or opened by Acrobat must have less pages than that. If you were to create a native x64 PDF application you could add more pages, but the resulting files wouldn't open at all in 32-bit apps." This forum entry goes on to say, "Architecturally there is only one limit in the PDF standard: the overall file size must be below ~10GB as the cross-reference tables which define the PDF structure use 10 bits."
The preceding paragraph offers a generous view of the potential size for a PDF. Many commentators argue that the limit for practicality is lower than those stated above. What matters is whether you can open a given PDF in any reasonable application, including Acrobat and Adobe Reader, mentioned above. Online forums also include reports like these examples: "It seems that the iPad has a limit of 30MB for displaying PDF files," and "users of GoodReader have reported flawless performance with files over 1 gig in size." The practical limits imposed by applications might also include limits set by indexers if the PDF includes searchable text.
Self-identification of chronological versions of PDF: Identification of chronological versions of PDF can be given in two places in a PDF file. All PDF files should have a version identified in the header with the 5 characters %PDF– followed by a version number of the form 1.N, where N is a digit between 0 and 7 or a version number of 2.0. For example, PDF 1.7 would be identified as %PDF–1.7. However, beginning with PDF 1.4, a conforming PDF writer may use the Version entry in the document Catalog to override the version specified in the header. The location of the Catalog within the file is indicated in the Root entry of the file trailer/footer. This override feature was introduced to facilitate the incremental updating of a PDF by simply adding to the end of the file. As a result, it is necessary to locate the Catalog within the file to get the correct version number. Unless the PDF is "linearized," in which case the Catalog is up front, this will require reading the trailer and then using the reference there to locate the Catalog, which will typically be compressed. This has practical implications because format identification tools, including DROID, typically look for particular characters at the beginning of a file (i.e., in the header), to permit identification with minimal effort. DROID can look for characters at the end of the file, but is not able to follow an indirect reference or decompress file contents. When the version number is not the same in the header and the Catalog, there is potential for format identification errors.
Compression of "stream objects" in PDF files: Stream objects in a PDF file are often compressed. A number of compression schemes are supported for PDF files, indicated by Filter values defined in the specification. Filter names correspond to the decoding/decompression that must be applied to recover the original data. Filters can be combined into pipelines. The Filters listed below are permitted in generic PDF files. However, some filters are not permitted in the "subset standards for PDF," such as PDF/A, PDF/X, and PDF/E.
The Crypt filter (introduced in PDF 1.5) can be used to specify the encryption algorithm that has been applied to a datastream. Many of the encryption algorithms supported in earlier chronological versions of PDF are now deprecated.
Tagged PDF: The concept of a tagged PDF was introduced in PDF 1.4. In addition to the content tree that is part of any PDF, a tagged PDF also has a structure tree. Tags provide a logical structure that governs how the document content is presented through assistive technology. Each tag identifies the associated content element, for example paragraph <P>, heading level three <H3>, list item <LI>, image <Figure>, or table data cell <TD>. The order of the tags defines the reading order. PDF Reference (third edition), which specifies PDF 1.4, indicates that Tagged PDF is a stylized use of PDF that uses a set of standard structure types and attributes that allow page content (text, graphics, and images) to be extracted and reused for other purposes. Tagged PDFs follow a set of rules for representing text in the page content so that characters, words, and text order can be determined reliably for use by tools that perform operations such as:
Many PDF files are created by "printing to PDF" or other methods that to not create tag structures. In general, the logical structure of a document is only represented in a PDF file if the creator or a process during creation takes steps to incorporate structural tagging. See What is Tagged PDF? from 2004, in which Duff Johnson said, "Tags may be generated automatically for any PDF file using Acrobat 6.0 Professional, but unless the document is very simple indeed, automated tagging alone is unlikely to produce satisfactory results, and is certainly not a quick-fix for compliance with Section 508." Some applications for which the native format is inherently structured appropriately have specialized exports that can create tagged PDFs that represent that structure. For example, according to the description of exporting to accessible PDF from Framemaker (desktop publishing software), "Tagged Adobe PDF provides the following capabilities:
The guidance on accessibility using Adobe InDesign encourages users to "apply accessibility features within your InDesign document, rather than having to make major changes in Adobe Acrobat software. PDF tags, alt tags, and the content order you assign stay with the document as you revise it."
Adapted from PDF Reference, Third Edition: The origins of PDF and the Adobe Acrobat product family date to early 1990. At that time, the PostScript page description language was rapidly becoming the worldwide standard for the production of the printed page. PDF builds on the PostScript page description language by layering a document structure and interactive navigation features on PostScript's underlying imaging model, providing a convenient, efficient mechanism enabling documents to be reliably viewed and printed anywhere.
See Wikipedia entry on History of the Portable Document Format (PDF) and descriptions for chronological versions for later history.
The agenda for the ISO TC 171 SC 2 “PDF Week” – Spring, 2019 indicates that a "dated revision" of ISO 32000-2 (PDF 2.0) is in process and that several new editions for the subset standards are waiting for that revision.