Difference between revisions of "PDF/A in PDS4 - A Primer"

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:; PDF/A-1a — Level A (Accessible) Conformance
 
:; PDF/A-1a — Level A (Accessible) Conformance
 
:* This is the much preferred conformance level for PDS4.
 
:* This is the much preferred conformance level for PDS4.
:* Not only does this level better accommodate people with disabilities, it's also recommended over Level B "for an exact text searchability, text extraction, and for the reuse of content."<ref name="PDFA FAQ">McAlearney, S. (n.d.). [https://www.pdfa.org/pdfa-faq/ PDF/A FAQ]. Retrieved July 11, 2017, from https://www.pdfa.org/pdfa-faq/</ref>
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:* Not only does this level better accommodate people with disabilities, it's also recommended over Level B "for an exact text searchability, text extraction, and for the reuse of content."<ref name="PDFA FAQ">McAlearney, S. (n.d.). [https://www.pdfa.org/pdfa-faq/ PDF/A FAQ]. Retrieved July 11, 2017.</ref>
  
 
Note that it's not immediately apparent whether a PDF file conforms to a standard. All PDF documents are defined by the same file format, after all, and thus they all use the <code>.pdf</code> filename extension. A document that is intentionally standard-compliant should, however, have XMP metadata which specifies its standard. A file which claims to conform to PDF/A-1a, for example, will include the following tags in the <code>pdfaid</code> namespace:
 
Note that it's not immediately apparent whether a PDF file conforms to a standard. All PDF documents are defined by the same file format, after all, and thus they all use the <code>.pdf</code> filename extension. A document that is intentionally standard-compliant should, however, have XMP metadata which specifies its standard. A file which claims to conform to PDF/A-1a, for example, will include the following tags in the <code>pdfaid</code> namespace:

Revision as of 19:24, 11 July 2017

PDF/A Overview

The Portable Document Format (PDF) is a broad, complex file format. The International Organization for Standardization (ISO) has created various PDF subset standards which are specialized for different uses. The PDF/A standard (ISO 19005) is designed for long-term preservation. There are three iterations of the PDF/A standard so far: PDF/A-1, PDF/A-2, and PDF/A-3. Versions of PDF/A retain backward but not forward compatibility (e.g., a document that complies with PDF/A-2 will also conform to PDF/A-3, but not necessarily to PDF/A-1). PDS4 requires compliance specifically to the first version, PDF/A-1.[1]

PDF/A-1

PDF/A-1 (ISO 19005-1) is based on PDF Version 1.4, and imposes further specifications.

PDF/A-1 files must include:
  • Embedded fonts [2]
  • Device-independent color [2]
  • XMP metadata [2]
    • XMP (Extensible Metadata Platform) is an ISO-standardized metadata model created by Adobe. A PDF file contains its metadata in <x:xmpmeta> tags in XML-based syntax. PDF/A-1 specifies information to be included in this structure, using both predefined XMP schemas and industry-specific XMP extension schemas.
    • From PDFlib: "PDF/A-1 requires XMP for identifying conforming files and supports custom metadata through XMP extension schemas. XMP support in PDF/A-1 is based on the XMP 2004 specification."[3]
PDF/A-1 files may not include:
  • Encryption [2]
  • LZW Compression [2]
  • Embedded files [2]
  • External content references [2]
  • PDF Transparency [2]
  • Multi-media [2]
  • JavaScript [2]

There are two levels of conformance:

PDF/A-1b — Level B (Basic) Conformance
  • This is the minimum conformance level required for PDS4.
PDF/A-1a — Level A (Accessible) Conformance
  • This is the much preferred conformance level for PDS4.
  • Not only does this level better accommodate people with disabilities, it's also recommended over Level B "for an exact text searchability, text extraction, and for the reuse of content."[4]

Note that it's not immediately apparent whether a PDF file conforms to a standard. All PDF documents are defined by the same file format, after all, and thus they all use the .pdf filename extension. A document that is intentionally standard-compliant should, however, have XMP metadata which specifies its standard. A file which claims to conform to PDF/A-1a, for example, will include the following tags in the pdfaid namespace:

<pdfaid:part>1</pdfaid:part>
<pdfaid:conformance>A</pdfaid:conformance>

A PDF reader may make note of this designation. Adobe Acrobat, in particular, displays a blue banner which states that the file "claims compliance with the PDF/A standard." Regardless, this claim of compliance must be validated. A non-compliant document may incorrectly have metadata indicating compliance; conversely, compliance metadata may be missing from an otherwise compliant document.

See Also

External Links

General
Backend

References

  1. Policy on Formats for PDS4 Data and Documentation. (2014, June 30). The Planetary Data System. From PDS Policies
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 19005-1 FAQ. (2006, July 10). PDF/A Joint Working Group. From NPES PDF/A FAQ
  3. XMP Metadata. (n.d.). Retrieved July 07, 2017.
  4. McAlearney, S. (n.d.). PDF/A FAQ. Retrieved July 11, 2017.