Sustainability of Digital Formats: Planning for Library of Congress Collections

Part 1 of the PDF/A ISO standard [ISO 19005-1:2005] is a constrained form of Adobe PDF version 1.4, intended to be suitable for long-term preservation of page-oriented documents for which PDF is already being used in practice. Level A conformance (PDF/A-1a) indicates complete compliance with the ISO 19005-1 requirements, including those related to structural and semantic properties of documents. Level B conformance (PDF/A-1b) indicates minimal compliance to ensure that the rendered visual appearance of a conforming file is preservable over the long term.

See PDF/A-1.

PDF/A-1a requires representation of the logical structure of the document as specified in section 6.8 of ISO 19005-1. The standard recommends that creators "should attempt to capture a document's logical structure hierarchy to the finest granularity possible." Nevertheless, the standard also indicates that "PDF/A-1 writers should not add structural or semantic information that is not explicitly or implicitly present in the source material solely for the purpose of achieving conformance." Hence, the logical structure of a document will only be represented to the degree the creator or process during creation takes steps to incorporate relevant structural tagging.

In practice, many PDF/A-1a files are not tagged to support accessibility. See Accessibility: What PDF/A-1a Really Means, an article from 2010. The PDF/UA standard for PDF Enhancement for Accessibility was initially developed as a set of additional requirements above those included in PDF/A-1a, although by the time it became a standard, it was specified as extra requirements on PDF 1.7 (ISO 32000-1) and thus closer to PDF/A-2a.