PDF/A is an ISO-standardized version of the Portable Document Format (PDF) specialized for the digital preservationof electronic documents.[3]
PDF/A differs from PDF by prohibiting features ill-suited to long-term archiving, such as font linking (as opposed to font embedding).[3]
The ISO requirements for PDF/A file viewers include color management guidelines, support for embedded fonts, and auser interface for reading embedded annotations.
Background
PDF is a standard for encoding documents in an “as printed” form that is portable between systems. 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 defining the use of the Portable Document Format (PDF) for archiving documents.[5] 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 ensure that those documents would be able to be retrieved and rendered with a consistent and predictable result in the future.[6] This need exists in a wide variety of government and industry areas world-wide, including legal systems, libraries, newspapers, and regulated industries.[7]
Description
The PDF/A 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 using various software 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 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 may include annotations (e.g. hypertext links) that link to external documents.[6]
Other key elements to PDF/A conformance include:[8][9][10]
- Audio and video content is forbidden.
- JavaScript and executable file launches are 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 required.
- External content references are forbidden.
- LZW and JPEG2000 image compression models 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 are allowed 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 allows embedding of PDF/A files, facilitating the 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 documents.
- The use of XML-based XML Forms Architecture (XFA) forms is forbidden in PDF/A. (XFA form data may be preserved in a PDF/A-2 file by moving from XFA key to the Names tree that itself is the value of the XFAResources key of the Names dictionary of the document catalog dictionary.)
- Interactive PDF forms – Form fields must have an appearance dictionary associated with the field’s data. The appearance dictionary shall be used when rendering the field.
*Source: http://en.wikipedia.org/?title=PDF/A
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