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|Type code||'PDF ' (including a single space)|
PDF/A differs from PDF by omitting features ill-suited to long-term archiving, such as font linking (as opposed to font embedding). (Similarly, the PDF/X file format is specially adapted to digital printing and graphic arts.)
PDF/A-1 is based on the PDF Reference Version 1.4 from Adobe Systems Inc. (implemented in Adobe Acrobat 5 and later versions) and is defined by ISO 19005-1:2005, an ISO Standard that was published on October 1, 2005: Document Management – Electronic document file format for long term preservation – Part 1: Use of PDF 1.4 (PDF/A-1)
PDF/A-2 is based on ISO 32000-1 – PDF 1.7 and is defined by ISO 19005-2:2011, published on June 20, 2011 under the formal name Document management – Electronic document file format for long-term preservation – Part 2: Use of ISO 32000-1 (PDF/A-2). PDF/A-2 is a very recent standard and is not widely used.
PDF/A-3 is based on ISO 32000-1 – PDF 1.7 and is defined by ISO 19005-3:2012, published on October 15, 2012 under the formal name Document management -- Electronic document file format for long-term preservation -- Part 3: Use of ISO 32000-1 with support for embedded files (PDF/A-3).
|Part||Name||Formal name||Release date||Standard||Based on PDF version|
|Part 1||PDF/A-1||Use of PDF 1.4 (PDF/A-1)||2005||ISO 19005-1||PDF 1.4 (Adobe Systems, PDF Reference third edition, 2001)|
|Part 2||PDF/A-2||Use of ISO 32000-1 (PDF/A-2)||2011||ISO 19005-2||PDF 1.7 (ISO 32000-1:2008)|
|Part 3||PDF/A-3||Use of ISO 32000-1 with support for embedded files (PDF/A-3)||2012||ISO 19005-3||PDF 1.7 (ISO 32000-1:2008)|
PDF is a standard for encoding documents in an "as printed" form that is portable between systems and is widely used for distribution and archiving of documents. However, the suitability of a PDF file for archival preservation depends on options chosen when the PDF is created: most notably, whether to embed the necessary fonts for rendering the document; whether to use encryption; and whether to preserve additional information from the original document beyond what is needed to print it.
PDF/A was originally a new joint activity between The Association for Suppliers of Printing, Publishing and Converting Technologies (NPES) and the Association for Information and Image Management, to develop an International standard to define the use of the Portable Document Format (PDF) for archiving and preserving documents. The goal was to address the growing need to electronically archive documents in a way that would ensure preservation of their contents over an extended period of time, and would further ensure that those documents would be able to be retrieved and rendered with a consistent and predictable result in the future. This need exists in a growing number of international government and industry segments, including legal systems, libraries, newspapers, and regulated industries.
The Standard does not define an archiving strategy or the goals of an archiving system. It identifies a "profile" for electronic documents that ensures the documents can be reproduced exactly the same way in years to come. A key element to this reproducibility is the requirement for PDF/A documents to be 100% self-contained. All of the information necessary for displaying the document in the same manner every time is embedded in the file. This includes, but is not limited to, all content (text, raster images and vector graphics), fonts, and color information. A PDF/A document is not permitted to be reliant on information from external sources (e.g. font programs and data streams), but is permitted to include annotations (e.g. hypertext links) that link to external documents.
- Audio and video content is forbidden.
- All fonts must be embedded and also must be legally embeddable for unlimited, universal rendering. This also applies to the so-called PostScript standard fonts such as Times or Helvetica.
- Colorspaces specified in a device-independent manner.
- Encryption is forbidden.
- Use of standards-based metadata is mandated.
- External content references are forbidden.
- LZW and JPEG2000 image compressions are forbidden in PDF/A-1, but JPEG 2000 compression is allowed in PDF/A-2.
- Transparent objects and layers (Optional Content Groups) are forbidden in PDF/A-1, but they are supported in PDF/A-2.
- Provisions for digital signatures in accordance with the PAdES (PDF Advanced Electronic Signatures) standard are supported in PDF/A-2.
- Embedded files are forbidden in PDF/A-1, but PDF/A-2 offers the possibility to embed PDF/A files, allowing archiving of sets of PDF/A documents in a single file. PDF/A-3 allows embedding of any file format such as XML, CAD and others into PDF/A as complete archived objects.
Conformance levels and versions
The standard specifies two levels of compliance for PDF files:
- PDF/A-1a – Level A compliance in Part 1
- PDF/A-1b – Level B compliance in Part 1
PDF/A-1b has the objective of ensuring reliable reproduction of the visual appearance of the document.
PDF/A-1a includes all the requirements of PDF/A-1b and additionally requires:
- document structure must be included (hierarchy)
- Tagged PDF (use of alternative texts for images, tagging text spans and giving them an ID, replacement texts for symbols)
- Unicode character maps
- language specification.
PDF/A-1a objective is to ensure that document content can be searched and repurposed.
The requirements for Level A conformance place greater responsibilities on writers preparing conforming files, but these requirements allow for a higher level of document preservation service and confidence over time. Level A conformance also facilitates the accessibility of conforming files for physically impaired users.
According to the specification, the following terms are recommended when referring to the ISO 19005-1:2005 specification when the full ISO name is not being used:
- PDF/A – a synonym for the ISO 19005 family of standards
- PDF/A-1 – a synonym for ISO 19005-1
- PDF/A-1a – a synonym for ISO 19005-1 Level A conformance
- PDF/A-1b – a synonym for ISO 19005-1 Level B conformance
PDF/A-2 is the second part to the standard. PDF/A-2 address some of the new features added with versions 1.5, 1.6 and 1.7 of the PDF Reference. PDF/A-2 should be backwards compatible, i.e. all valid PDF/A-1 documents should also be compliant with PDF/A-2. However PDF/A-2 compliant files will not necessarily be PDF/A-1 compliant.
Part 2 of the PDF/A Standard is based on a more recent version, PDF 1.7 (ISO 32000-1), rather than PDF 1.4 and offers a number of new features:
- JPEG2000 image compression
- support for transparency effects and layers
- embedding of OpenType fonts
- provisions for digital signatures in accordance with the PDF Advanced Electronic Signatures – PAdES standard
- possibility to embed PDF/A files in PDF/A-2 for archiving of sets of documents as individual documents in a single file.
Part 2 defines three conformance levels: PDF/A-2a, PDF/A-2b and a new conformance level PDF/A-2u. PDF/A-2u represents Level B conformance (PDF/A-2b) with the additional requirement that all text in the document have Unicode mapping.
PDF/A-3 (ISO 19005-3:2012. Part 3) allows embedding of arbitrary file formats (such as XML, CSV, CAD, wordprocessing documents, spreadsheet documents and others) into PDF/A as complete archived objects.
The PDF/A-3 specification was published on October 17, 2012.
A PDF/A document can be identified as such through PDF/A-specific metadata located in the "http://www.aiim.org/pdfa/ns/id/" namespace. However, claiming to be PDF/A and being so are not necessarily the same :
- A PDF document can be PDF/A-compliant, except for its lack of PDF/A metadata. This may happen for instance with documents that were generated before the definition of the PDF/A standard, by authors aware of features that present long-term preservation issues.
- A PDF document can be identified as PDF/A, but may incorrectly contain PDF features not allowed in PDF/A; hence, documents which claim to be PDF/A-compliant should be tested for PDF/A compliance.
PDF/A viewer mode
The PDF/A specification also states some requirements for a conforming PDF/A reader, which must
- ignore any data that are not described by the PDF and PDF/A standards;
- ignore any linearization information provided by the file;
- only use the embedded fonts (rather than any locally available, substituted or simulated fonts);
- only display using the embedded colour profile;
- ensure that form fields do not change the rendered presentation and are rendered without regard to the form data;
- ensure that annotations are rendered consistently.
Some PDF viewers, e.g., Adobe Reader 9, will by default switch into a special "PDF/A viewing mode" to fulfill these requirements whenever a document declares in its metadata that it is PDF/A compliant. This may also alert the user that this mode has been activated, and disable functions for changing the document.
As a PDF/A document must embed all fonts that it uses, a PDF/A file will often be bigger than an equivalent PDF file that does not have the fonts embedded.
The use of transparency is forbidden in PDF/A-1. The majority of PDF generation tools that allow for PDF/A document compliance, such as the PDF export in OpenOffice.org or PDF export tool in Microsoft Office 2007 suites, will also make any transparent images in a given document non-transparent. That restriction was removed in PDF/A-2.
- ISO (2005). "ISO 19005-1:2005 – Document management – Electronic document file format for long-term preservation – Part 1: Use of PDF 1.4 (PDF/A-1)". Retrieved 2011-07-06.
- ISO (2011-06-20). "ISO 19005-2:2011 – Document management – Electronic document file format for long-term preservation – Part 2: Use of ISO 32000-1 (PDF/A-2)". Retrieved 2011-07-06.
- ISO 19005-3:2012 - Document management -- Electronic document file format for long-term preservation -- Part 3: Use of ISO 32000-1 with support for embedded files (PDF/A-3), retrieved 2012-10-23
- "PDF/A – A Look at the Technical Side". Retrieved 2011-07-06.
- "PDF/A-2 Standard Published by ISO! The New Standard Includes Great Technical Enhancements.". 2011-07-01. Retrieved 2011-07-06.
- Frequently Asked Questions (FAQs) – ISO 19005-1:2005 – PDF/A-1, Date: July 10, 2006 (PDF), 2006-07-10, retrieved 2011-07-06
- Improved PDF/A-1b, retrieved 2012-09-26
- PDF/A-2, PDF for Long-term Preservation, Use of ISO 32000-1 (PDF 1.7), Library of Congress, retrieved 2012-09-26
- PDF Association Arranges Its First Seminar on PDF/A to Include Standards 1 to 3, 2012-03-29
- PDF/A-3 published by ISO
- AIIM's PDF/A Committee
- PDF Standards – The PDF/A Standards Wiki
- PDF/A Competence Center
- First International PDF/A Conference in Amsterdam
- Third International PDF/A Conference in Berlin
- Fourth International PDF/A Conference in Rome
- PDF/A 101: An Introduction – presentation from the First International PDF/A Conference in Amsterdam
- Format description for PDF/A-1, PDF for Long-term Preservation, Use of PDF 1.4 – from digitalpreservation.gov
- White Paper: PDF/A – The Basics – from PDF Tools AG
- PDF/A in a Nutshell 2.0 – published by PDF Association (2013)