PDF/A in PDS4 - A Primer

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PDS4-compliant formats for documents are “flat UTF-8” text, PDF/A-1a (which is preferred), or PDF/A-1b.[1] This page will provide an overview of PDF/A-1a and PDF/A-1b in PDS4.

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.[2]


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 [3]
  • Device-independent color [3]
  • XMP metadata [3]
    • 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."[4]

PDF/A-1 files may not include:

  • Encryption [3]
  • LZW Compression [3]
    • LZW is a proprietary, somewhat outmoded lossless data compression algorithm.
    • Flate compression, which is more efficient and is in the public domain, is generally used instead of LZW.
    • ZIP is allowed [5]
    • JPEG is allowed,[5] but not JPEG2000 [6]
      • Note that though the ISO standard recommends lossless compression, lossy compression isn't prohibited.[6]
  • Embedded files [3]
    • This refers specifically to embedded file streams, which are used to embed contents of an external file within the body of the PDF.[7]
    • Not to be confused with "embedded" images.[7] PDS4 Standards Reference states that figures may be "embedded" in PDFs;[1] this isn't prohibited by PDF/A-1.
    • From veraPDF's validation rules:

A file specification dictionary, as defined in PDF 3.10.2, shall not contain the EF key [...] This key is used to encapsulate files containing arbitrary content within a PDF file. The explicit prohibition of EF key has the implicit effect of disallowing embedded files that can create external dependencies and complicate preservation efforts.[8]

  • External content references [3]
  • PDF Transparency [3]
  • Multi-media [3]
  • JavaScript [3]
    • From veraPDF: "JavaScript actions permit an arbitrary executable code that has the potential to interfere with reliable and predictable rendering."[8]

There are two levels of conformance:

PDF/A-1b — Level B (Basic) Conformance
  • This is the minimum conformance level required for PDS4.
  • Encompasses ISO 19005-1 requirements "regarding the visual appearance of electronic documents, but not their structural or semantic properties" [9]
PDF/A-1a — Level A (Accessible) Conformance
  • This is the much preferred conformance level for PDS4.
  • Encompasses all requirements of ISO 19005-1 [9]
  • Recommended over Level B "for an exact text searchability, text extraction, and for the reuse of content."[5]

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:


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



  1. 1.0 1.1 NASA, JPL. (2017). Planetary Data System Standards Reference Version 1.8.0 (JPL D-7669, Part 2). Pasadena, CA: Jet Propulsion Laboratory.
  2. Policy on Formats for PDS4 Data and Documentation. (2014, June 30). The Planetary Data System. From PDS Policies
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 19005-1 FAQ. (2006, July 10). PDF/A Joint Working Group. From NPES PDF/A FAQ
  4. XMP Metadata. (n.d.). Retrieved July 07, 2017.
  5. 5.0 5.1 5.2 McAlearney, S. (n.d.). PDF/A FAQ. Retrieved July 11, 2017.
  6. 6.0 6.1 Isaacs, D. (2012, May 27). Re: What are the permitted compression techniques for PDF/A-1? Retrieved July 12, 2017, from https://forums.adobe.com/thread/1012639. Various discussion board replies by Adobe employee.
  7. 7.0 7.1 Van der Knijff, V. (2013, January 9). What do we mean by “embedded” files in PDF? [Web log post]. Retrieved July 12, 2017.
  8. 8.0 8.1 VeraPDF Consortium. (2016, December 23). PDF/A-1 validation rules. Retrieved July 12, 2017.
  9. 9.0 9.1 ISO 19005-1:2005(en) Document management — Electronic document file format for long-term preservation — Part 1: Use of PDF 1.4 (PDF/A-1). (2005). Geneva: ISO. Preview only, retrieved from https://www.iso.org/obp/ui/#iso:std:iso:19005:-1:ed-1:v2:en.