Sustainability of Digital Formats: Planning for Library of Congress Collections

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PDF/A, PDF for Long-term Preservation

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name ISO 19005. Document management - Electronic document file format for long-term preservation

PDF/A is a family of ISO standards for constrained forms of Adobe PDF intended to be suitable for long-term preservation of page-oriented documents for which PDF is already being used in practice. The PDF/A standards are developed and maintained by a working group with representatives from government, industry, and academia and active support from Adobe Systems Incorporated. The working group is WG 5 of Technical Committee ISO/TC 171, Document management applications, Subcommittee SC 2, Application issues [ISO TC171/SC2/WG5]. This group works in cooperation with: ISO/TC130, Graphics technology; ISO/TC42, Photography; and ISO/TC46/SC11, Information and documentation, Archives/records. management.

PDF/A-1, the first PDF/A standard [ISO 19005-1:2005], was based on PDF version 1.4 and published in 2005. PDF/A-2 as defined in ISO 19005-2:2011, extends the capabilities of PDF/A-1 and is based on PDF version 1.7 (as defined in ISO 32000-1). PDF/A-3 adds a single and highly significant feature to its predecessor PDF/A-2, to permit the embedding within a PDF/A file a file, or files, in any other format, not just other PDF/A files (as permitted in PDF/A-2). The intent expressed by many proponents is that the embedded files not be considered part of the archival payload. However, use cases are emerging where the embedded files would likely warrant preservation by archival institutions.

PDF/A attempts to maximize:

  • Device independence
  • Self-containment
  • Self-documentation

The constraints include:

  • Audio and video content are forbidden
  • Javascript and executable file launches are prohibited
  • All fonts must be embedded and also must be legally embeddable for unlimited, universal rendering
  • Colorspaces specified in a device-independent manner
  • Encryption is disallowed
  • Use of standards-based metadata is mandated

The PDF/A standards define levels of conformance: conformance level A satisfies all requirements in the specification; level B and level U are lower levels of conformance, still satisying the requirements of ISO 19005 regarding the visual appearance of electronic documents, but less demanding as to representation of structural or semantic properties.

Production phase A final-state format for delivery to end users and long-term preservation of the document as disseminated to users.
Relationship to other formats
    Subtype of PDF, Portable Document Format
    Has subtype PDF/A-1, PDF for Long-term Preservation, Based on PDF 1.4
    Has subtype PDF/A-2, PDF for Long-term Preservation, Use of ISO 32000-1 (PDF 1.7) (PDF/A-2)
    Has extension PDF/A-3, PDF for Long-term Preservation, Use of ISO 32000-1 (PDF 1.7) with Embedded Files.

Local use Explanation of format description terms

LC experience or existing holdings LC was represented on the working group for the original PDF/A standard and continues to be active in the development of new versions.
LC preference

The Library of Congress expresses preferences for formats for content for its collections through through two venues:

  • the "Best Edition" specification from the U.S. Copyright Office in Circular 7b. Rev: 08 ⁄ 2010 of Circular 7b lists formats acceptable for mandatory deposit of Electronic Serials available only online, in order of preference. For page-oriented renditions, PDF/A appears first on the list. Other forms of PDF are acceptable, preferably with searchable text. [Note: When this Circular was published, PDF/A-2 and PDF/A-3 did not exist.]
  • The Library of Congress Recommended Format Statement (RFS) includes PDF/A as a preferred format for textual works in digital form, electronic serials, digital musical compositions, and accompanying image/text files for digital audio. The RFS also includes PDF/A as an acceptable format for other graphic images - digital. The RFS list does not distinguish between versions of PDF/A. In general, PDF/A-1 and PDF/A-2 are preferred formats for page-oriented textual (or primarily textual) documents when layout and visual characteristics are more significant than logical structure. Note that, for PDFs based on page images digitized by scanning, the source images are considered the master format if available. PDFs created from those images may be optimized for access convenience rather than sustainability.

Sustainability factors Explanation of format description terms


A family of open standards. Developed by a working group (WG 5) under ISO/TC171/SC2, the subcommittee for Document Management Applications, Application Issues, for which AIIM (The Association for Information and Image Management) acts as secretariat until August 2016. WG5 is a Joint Working Group, which also includes ISO/TC 46 SC11, Archives/records Management, ISO/TC 130, Graphics Technology, and ISO/TC 42, Photography.


ISO 19005-1:2005. Document management -- Electronic document file format for long-term preservation -- Part 1: Use of PDF 1.4 (PDF/A-1). The standard cannot be used without PDF Reference, Third Edition, Version 1.4, which it uses as a normative reference.

International Standard ISO 19005-2:2011. Document management -- Electronic document file format for long-term preservation -- Part 2: Use of ISO 32000-1 (PDF/A-2). The standard cannot be used without ISO 32000-1, which it uses as a normative reference.

International Standard ISO 19005-3:2012. Document management -- Electronic document file format for long-term preservation -- Part 2: Use of ISO 32000-1 with support for embedded files (PDF/A-3). The standard cannot be used without ISO 32000-1, which it uses as a normative reference.


Since the initial PDF/A standard was published in late 2005, tools for creation, conversion, and validation have been reaching the market steadily. Adobe's own Acrobat Professional 7.0 allowed saving files in a form compliant with the draft standard. Acrobat 8 and later versions support the standard as published. Microsoft Office 2007 supported creation of PDF/A files through Save as PDF, an add-on module. Open Office introduced support for PDF/A in release 2.4 (in early 2008).

Several commercial companies with products aimed at large enterprises, have produced products supporting the creation, migration, and validation of PDF/A files: Apago, Inc., Visioneer (for scanning paper to PDF/A), Callas Software, Compart Systemhaus, Luratech, Nuance, PDF Tools AG. Many of these companies are based in Europe, where the growing requirements from the EU for use of digital formats that are formal (preferably ISO) standards has produced more market pressure than in the U.S. Starting with version 0.93 (released in January 2007), the widely used open source FOP (Formatting Object Processor, based on the W3C's XSL-FO standard) from Apache, has support for the minimal PDF/A profile, PDF/A-1b. A list of supporting products, compiled by AIIM, based on information supplied by vendors, can be found through the Internet Archive's Wayback Machine

The PDF/A Competence Center is a source of news and information about PDF/A. For example, 4th International PDF/A Conference Proceedings includes some articles about adoption of and support for PDF/A. In March 2009, the center issued a press release about a survey, titled Nearly all archiving projects use PDF/A .

The standards development process involved active participation on behalf of communities whose endorsement or adoption would create significant momentum for wider adoption in the sense of requirement or preference for PDF/A over generic PDF for archival deposit or submission. Important groups are government agencies and legislative and judicial institutions. Adobe reported migration of legacy "report silos" at several (un-named) financial institutions at a meeting of the European DLM (Document Lifecycle Management) Forum in Helsinki in November 2006. An increasing number of libraries and other archival institutions are recommending or requiring PDF/A. For pragmatic reasons, when PDF/A is mandated, PDF/A-1b is usually acceptable. Full PDF/A-1a compliance, with tagged document structure, is hard to achieve except in a workflow that anticipates that objective from initial document creation. A few examples of libraries and archives recommending or mandating PDF/A are: Virginia Tech for electronic theses; National Archives of Norway; University of Texas Libraries (for textual documents deposited in a digital repository).

Within the U.S. Government, there is an increasing level of encouragement for the use of PDF/A. The U.S. National Archives and Record Administration (NARA) has participated actively in the development of PDF/A and its 2014 guidance for transfer of records by government agencies lists PDF/A as a preferred format for textual documents, presentations, posters, and scanned text. The United States Patent and Trademarks Office (USPTO) has requirements for PDFs that it accepts for electronic filing; the requirements are based on the PDF/A specification. Documents conforming to PDF/A-1 meet the USPTO requirements. According to an announcement available on the PACER (Public Access to Court Electronic Records -- for U.S. Federal Courts) web site in February 2011, "The Judiciary is planning to change the technical standard for filing documents in the Case Management and Electronic Case Filing (CM/ECF) system from PDF to PDF/A." In July 2012, a press release by Adlib indicated that the U.S. Department of State had replaced its cable system based on ASCII text with one based on PDF/A.

A list of entities recommending or requiring use of PDF/A was found at from Adobe between 2010 and early 2013 (link now via Internet Archive at

Although many applications claim to create PDF/A files, experience has shown that many PDF documents that identify themselves as PDF/A, are not fully compliant. In late 2014, a project to develop an open source conformance checker for PDF/A was launched under the auspices of the EU-funded PREFORMA project by the VeraPDF Consortium. In Spring 2015, results from the design phase were made available at PREFORMA: PDF/A Conformance Checker.

    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, and hence in the ISO 19005 specifications by reference. As an ISO standard, the compliance of approved parts of ISO 19005 with the ISO/IEC/ITU common patent policy has been vetted.

A summary of relevant information on the Adobe Web site in September 2010 (see follows.

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); 5,737,599 (filed 1995-12-07, granted 1998-04-07); 5,781,785 (filed 1995-09-26, granted 1998-07-14); 5,819,301 (filed 1997-09-09, granted 1998-10-06); 6,028,583 (filed 1998-01-16, granted 2002-02-22); 6,289,364 (filed 1997-12-22, granted 2001-09-11); 6,421,460 (filed 1999-05-06, granted 2002-07-16). Patent 5,860,074 (filed 1997-08-14, granted 1999-01-12) 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).

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. PDF/A does not permit encryption.
Self-documentation Support for embedding any form of metadata for a document is extremely good. Use of XMP is mandatory for basic descriptive and identifying metadata. Other XMP metadata packages can be embedded.
External dependencies PDF/A is constrained to avoid external dependencies. All necessary fonts must be embedded.
Technical protection considerations PDF/A does not permit encryption.

Quality and functionality factors Explanation of format description terms

Normal rendering

Good support is possible, but not guaranteed. The PDF/A format does not preclude creating documents from scanned page images; such files do not necessarily support indexing of the document text or extraction of text for quotation. See Notes below for more on creating PDF/A documents by scanning.

Integrity of document structure The logical structure of a document is only represented in a PDF/A file if the creator or process during creation takes steps to incorporate structural tagging. The PDF/A standard recommends the representation of structural hierarchy
Integrity of layout and display PDF is designed to represent the layout of page-oriented documents.
Support for mathematics, formulae, etc. Can be represented by embedded graphics.
Functionality beyond normal rendering Annotations may be embedded. Bookmarks may be provided.

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension pdf
The standard does not indicate that a different extension should be used to distinguish PDF from PDF/A.
Internet Media Type See related format.  See PDF.
Magic numbers See related format.  See PDF.
Indicator for profile, level, version, etc. See note.  The standard specifies that the PDF/A version and conformance level of a file shall be specified using the PDF/A Identification extension schema defined in the standard. This schema has two mandatory elements: pdfaid:part (integer) and pdfaid:conformance (closed list of text values). For example a PDF/A-1b file should have the integer value 1 for pdfaid:part and the value "B" for pdfaid:conformance. See Notes below for example of tagging in this schema.
File signature See note.  PRONOM entry for fmt/95 provides signatures used by the DROID software to identify PDF/A files. This identification is based on the namespace declaration in the PDF/A Identification. See Notes below for an example of PDF/A Identification, including the namespace declaration, beginning xmlns.

Notes Explanation of format description terms


Each PDF/A standard is aligned to the fullest extent possible with the then current PDF/X standard.

A sample identification of part and conformance level is found within a mandatory metadata chunk for PDF/A Identification:
<rdf:Description rdf:about="" xmlns:pdfaid="">

In NARA 2014-04: Appendix A, Revised Format Guidance for the Transfer of Permanent Electronic Records – Tables of File Formats: 4.2 Scanned Text, the U.S. National Archives provides guidance on image quality when creating PDF/A files by scanning page images. Such guidelines improve visual legibility. However, the effectiveness of optical character recognition also depends heavily on the condition of the original document and the degree to which it employs small print, special fonts and complex layout. For documents that originate in electronic form and are primarily textual, it is almost always preferable to convert to PDF/A using a workflow that does not rely on printing and scanning (or an equivalent process using an intermediate raster image).

In his White Paper: How to Implement PDF/A, Duff Johnson suggests five general principles for PDF/A implementations and proposes three modes for PDF/A access: Advisory Mode; Strict PDF/A Mode; Strict PDF/A & Read-Only Mode. A sidebar shows how Adobe's Acrobat 9 did not follow his prescription and, when in its single PDF/A Mode prohibited certain tasks that should be permitted in some circumstances. In particular he notes that Adobe's handling of linearization (aka "fast web view") is unnecessarily restrictive. He argued that in his proposed Advisory Mode, linearization information should be usable, whereas Adobe ignored it. The specification for PDF/A-2 states,"Linearization shall be permitted but any linearization information present within a file should be ignored by conforming readers. NOTE As defined in ISO 32000-1:2008, Annex F, a PDF is not linearized if the value of the L key in the linearization dictionary does not match the actual length of the PDF file. This implies that an incremental update to a linearized PDF will render it non-linearized." Thus Duff Johnson's proposed Advisory mode would not be a fully "conforming reader." This situation suggests that, although "fast web view" is not strictly incompatible with PDF/A, many PDF/A tools are likely to behave as if it is.


Developed to address the issue that large bodies of official documents and important information are maintained in PDF, but that PDF is not suitable as an archival format. The Administrative Office of the U.S. Courts was a driving force in forming a U.S. Committee to initiate an ISO standard based on PDF. The development of ISO-19005-1 was under the joint auspices of AIIM and NPES (National Printing Equipment Suppliers).

Format specifications Explanation of format description terms

Useful references


Books, articles, etc.

Last Updated: Monday, 27-Feb-2017 09:55:28 EST