Sustainability of Digital Formats: Planning for Library of Congress Collections

PDF/E-1 is a constrained form of Adobe PDF intended to be suitable for exchange of 2D and 3D documentation in building and manufacturing workflows. The PDF/E-1 standard is maintained by a working group with representatives from government and industry, and active support from Adobe Systems Incorporated. The working group is WG 7 of Technical Committee ISO/TC 171, Document management applications, Subcommittee SC 2, Application issues [ISO TC171/SC2/WG5].

Among the objectives for PDF/E-1 were:

• Accurate printing of engineering drawings and predictable behavior in conforming viewers.
• Support for 3D artwork using the U3D format.
• Support for incorporating detailed engineering data.
• Support for review and commenting.
• Metadata associated with individual parts of a document, stored in XMP without compression.
• Secure document exchange in self-contained file, with digital signatures.

Constraints in PDF/E documents include:

• No external content or arbitrary actionable code in Postscript or JavaScript (except Javascript associated with 3D rendering) is permitted. No XFA-based forms are permitted.
• Audio and video clips must be entirely self-contained and embedded within a single stream.
• All colors must be specified in a device-independent manner, and consistently within a document.
• ICC-based color profiles must be embedded.
• All fonts used must be embedded and also must be legally embeddable for unlimited, universal rendering.
• All metadata must be represented in XMP. All metadata properties must use either the predefined schemas defined in Chapter 4 of the XMP Specification, or extension schemas that comply with the XMP Specification.

PDF/E-1 files may include:

• JavaScript associated with 3D content;
• Transparency and layers in graphics;
• Multimedia;
• Embedded files;
• Encryption, digital rights management (DRM), and digital signatures.

PDF/A-4e is the successor to PDF/E-1

An open international standard, published by ISO in 2008. Maintained by a working group (WG7) under ISO/TC 171 SC2, the subcommittee for Document Management Applications, Application Issues. From 2002 to 2016, AIIM (The Association for Information and Image Management) acted as secretariat and U.S. Technical Advisory Group (TAG) to ISO/TC 171 SC 2 (see AIIM | U.S. TAG to ISO/TC 171 from 2015). In 2017, the 3D PDF Consortium was approved by the American National Standards Institute (ANSI) as a standards developer and has assumed the role of secretariat and U.S. TAG Administrator for ISO/TC 171 SC 2 (see 3D PDF Consortium Approved by ANSI as US TAG Administrator for PDF ISO Standards).

The compilers of this resource have found little information as to the degree to which the formal PDF/E-1 standard has been used in practice, apart from the statement in PDF’s ISO-standardized subsets: a tour, a May 2018 slideshow from the PDF Association, that there has been almost no practical use of PDF/E-1. Comments welcome.

There are circumstances that suggest why adoption has not been widespread. PDF/E-1 only supports U3D for 3D content. Support for a different 3D encoding, Product Representation Compact (PRC), was introduced by Adobe in Acrobat 8.1 in 2007, before PDF/E-1 was published as ISO 24517-1:2008 or PDF 1.7 was published as ISO 32000-1:2008. PRC has advantages over U3D, particularly in supporting a very efficient specialized compression mechanism. Before PDF 1.7 was approved by ISO, Adobe documented an extension to PDF 1.7, PDF_1_7_ext03, which supported 3D content using PRC and a standardized way to represent geospatial locations, another feature of importance in some engineering contexts. For these reasons, engineering workflows were probably better served by what is often called "3D PDF" than by the formal PDF/E-1 standard.

In October 2018, the U.S. Department of Defense issued an update to its MIL-STD-31000 standard, which defines Technical Data Packages (TDP) as required in association with contracts. The purpose of a TDP is "to provide an authoritative technical description of an item which is clear, complete and accurate, and in a form and format adequate for its intended use." This update introduces the concept of the 3Di (3-Dimensional Intelligent) PDF viewable TDP. The definition for 3Di technical data is "A 3-dimensional viewable representation of an item provided in a widely available software format (e.g. ISO 32000-1 Portable Document Format (PDF)). This representation details the complete technical description of the required design configuration to include but not limited to geometry, topology, relationships, tolerances, attributes, metadata and other features necessary to define a component or assembly." It seems likely to the compilers of this resource that this standard will have an influence on the way the market for tools to create and render engineering documentation in PDF develops. Comments welcome.

A draft for PDF/E-2 has been circulated for comments as ISO 24517-2, to be based on PDF version 2.0 (as defined in ISO 32000-­2) rather than on PDF version 1.6. But the process stalled. Future ISO standards based on PDF 2.0 from PDFlib states "PDF/A-4e is targeted at the engineering community. It is slated as successor of the existing PDF/E-1 standard ISO 24517-1 which is based on PDF 1.6. The previous plan to define a new flavor PDF/E-2 as ISO 24517-2 has been canceled. Instead, PDF/A-4e adds RichMedia annotations for 3D content in U3D or PRC format to the base PDF/A-4 format." Comments welcome. In the meanwhile, there seems little reason for use of PDF/E-1, since PDFs based on PDF 2.0 (ISO 32000-2) can incorporate 3D in both U3D and PRC formats, and also take advantage of other features introduced with PDF 2.0 that are useful in engineering workflows, such as embedding of related files.

Support for embedding granular metadata for parts of a document is good. Use of XMP is mandatory and basic descriptive and identifying metadata uses properties defined in the standard.

Accessibility Features

PDF-E does not have specific features to support accessibility. See PDF/A-4 which states: "PDF/A-4 does not have a specific "a" level for accessibility. Instead, it encourages but doesn't mandate the inclusion of logical structure tags, and it requires Unicode mapping for all fonts. PDF/A-4 also recommends following the PDF/UA standard, which provides detailed guidelines and technical specifications for creating accessible tagged PDFs."

Self-identification of a PDF/E-1 file is found marked up in XML within a mandatory metadata chunk. A file would identify itself with XML equivalent to:
<rdf:Description rdf:about="" xmlns:pdfe="http://www.aiim.org/pdfe/ns/id/">
<pdfe:ISO_PDFEVersion>PDF/E-1</pdfe:ISO_PDFEVersion>
</rdf:Description>

ISO 24517-1:2008 states, "Neither the version number in the header of a PDF file, nor the value of the Versionkey in the Catalog of a PDF file shall be used in determining whether a file is in accordance with this part of this International Standard."

The PDF/E working group was formed in March 2004.  Participants included Adobe Systems Inc., Autodesk, Bentley Systems, Hewlett Packard, Intel, TerraGo (formerly Layton Graphics), Océ (a Canon subsidiary), Dell, U.S. Army Corps of Engineers, U.S. National Geospatial-Intelligence Agency, Boeing, Caterpillar, Honeywell, Xerox.   PDF/E (ISO 24517-1:2008) was approved by ISO in 2007 and published in 2008.

According to a 2018 presentation, PDF’s ISO-standardized subsets: a tour, the next part of PDF/A (ISO 19005), to be based on PDF 2.0, will incorporate a profile, PDF/A-4e, that will serve as the next generation of the PDF/E standard.