| Network Working Group | T. Hansen, Ed. |
| Internet-Draft | AT&T Laboratories |
| Intended status: Informational | L. Masinter |
| Expires: December 28, 2014 | M. Hardy |
| Adobe Systems | |
| June 26, 2014 |
PDF for an RFC Series Output Document Format
draft-hansen-rfc-use-of-pdf-00
This document discusses options and requirements for the PDF rendering of RFCs in the RFC Series, as outlined in RFC 6989. It also discusses the use of PDF for Internet Drafts, and available or needed software tools for producing and working with PDF.
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This Internet-Draft will expire on December 28, 2014.
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The RFC Series is evolving, as outlined in [RFC6949]. Future documents will use an archival format of XML with renderings in various formats, including PDF.
PDF has a wide range of capabilities and alternatives; not all PDFs are "equal". (See Appendix A for a brief history of PDF and its options.) For example, visually similar documents could be scanned or rasterized images, include text layout options, hyperlinks, embedded fonts, digital signatures. This document explains the options and also makes recommendations for choices, both for the RFC series and also Internet Drafts.
The PDF format and the tools to manipulate it are not as well known as those for other formats. This document discusses some of the processes for creating and using PDFs and both open source and commercial products.
NOTE: see https://github.com/masinter/pdfrfc for XML source, related files, and an issue tracker.
NOTE: this section is meant as an overview to give some background.
The RFC series has for a long time accepted Postscript renderings of RFCs, either in addition to or instead of the text renderings of those same RFCs. These have usually been produced when there was a complicated figure or mathematics within the document. For example, consider the figures and mathematics found in RFCs 1119 and RFC 1142, and compare the figures found in the text version of RFC 3550 with those in the Postscript version. The RFC editor has provided a PDF rendering of RFCs. Usually, this has been a print of the text file that does not take advantage of any of the broader PDF functionality, unless there was a Postscript version of the RFC, which would then be used by the RFC editor to generate the PDF.
In addition to PDFs generated and published by the RFC editor, the IETF tools community has also long supported PDF for Internet Drafts. Most RFCs start with Internet Drafts, edited by individual authors. The Internet drafts submission tool at https://datatracker.ietf.org/submit/ accepts PDF and Postscript files in addition to the (required) text submission and (currently optional) XML. If a PDF wasn't submitted for a particular version of an Internet Draft, the tools would generate one from the Postscript, HTML, or text.
This section lays out options and requirements for PDFs produced by the RFC editor for RFCs. There are two sections: "Visible" options are related to how the PDF appears when it is viewed with a PDF viewer. "Internal Structure" options affect the ability to process PDFs in other ways, but do not change the way the document looks.
In many cases, the choice of PDF requirements is heavily influenced by the utility of available tools to create PDFs. Most of the discussion of tooling is to be found in Section 4.
NOTE: each option in this section should outline the nature of the design choice, outline the pros and cons, and make a recommendation.
PDF supports rich visible layout of fixed-sized pages.
For a consistent 'look' of RFC and good style, the PDFs produced by the RFC editor should have a clear, easy-to-read style. They should print well on the widest range of printers, and look good on displays of varying resolution.
PDF files are laid out for a particular size of page, margins, and any headers and footers part of the layout. Recommendations or ideas for further study:
There is some advantage to having the PDF files look like the text or HTML renderings of the same document. There are several options even so. The PDF
(Note that numbers 1 and 2 are what are currently produced by the RFC Editor on their web site.)
Recommendation: the PDF rendition should look like the HTML rendition, at least in spirit -- for example, visually searching or scanning should be facilitated. The typeface and size for printing should be chosen.
A PDF may refer to a font by name, or it may use an embedded font. When a font is not embedded, a PDF viewer will attempt to locate a locally installed font of the same name. If it can not find an exact match, it will find a "close match". If a close match is not available, it will fallback to something. This is highly implementation dependent.
Recommendation: for consistent viewing, all fonts should be embedded.
In addition, since the HTML version of the document is being visually replicated, the font(s) chosen should have both variable width and constant width components, as well as bold and italic representations.
Few fonts have glyphs for the entire repertoire of Unicode characters; for this purpose, the PDF generation tool may need a set of fonts and a way of choosing them.
Recommendation: ... TBD ...
For readability, the main body text should be in a serif font and the headings in a sans-serif font.
Code, BNF, and other text could use a fixed-width font to aid in insuring alignment, e.g., in BNF.
PDF supports hyperlinks both to sections of the same document and to other documents.
Recommendation: All hyperlinks available in the HTML rendition of the RFC should also be visible and active in the PDF produced.
There are many things going on under the cover:
These all turn into requirements for the conversion tools that are used to generate the PDF rendering of the visible representation.
The areas we'll discuss are oriented around:
The contents of a PDF file can be represented in many ways. The PDF file could be generated:
All of these end up with the same visual representation of the output. However, each level has trade offs for auxiliary usage of the format. For example, the higher-level construct would allow you to search for the word "IETF" or phrases including the word "IETF"", whereas using word placement would only allow you to search for "IETF", and the other representations would not easily support search at all. As another example, when using an annotation tool to review a PDF file, it is harder to place a comment on a location within an image compared to attaching a comment to a given word. Attaching a comment to a set of words (such as a bracketed set of words) is easier when using higher-level constructs. Another example where higher-level constructs are needed are for accessibility purposes: text to speech needs the sentences to be presented as a whole and in the proper order.
Requirement: ... TBD ...
Unicode is being fully supported, so the RFC PDF format must similarly have full support for Unicode. While Unicode is not required by PDF, certain PDF profiles require its support.
Requirement: PDF files generated must have the full text, exactly as it appears in the original XML for text, or within SVG for images. (NOTE: What about text in image illustrations?)
Metadata encodes information about the document authors, the document series, date created, etc. using the RDF Dublin core (and other elements). Having this metadata within the PDF file allows it to be extracted by the rendering tools. It can also include additional information such as pointers to where the document can be found on the RFC Editor web site.
PDF supports embedded metadata using XMP. NOTE: Need a reference and explanation.
Recommendation: The PDFs generated should have all of the metadata from the XML version embedded directly as XMP metadata, including the author and date information, set the document series, and a URL for where the document can be retrieved.
The section structure of an RFC can be mapped into the PDF elements for the document structure. This will allow the bookmark feature of PDF readers to be used to quickly access sections of the document.
Requirement: The section structure of an RFC must be mapped into the PDF elements for the document structure. This would include section headings for the boilerplate sections such as the Abstract, Status of the Document, Table of Contents, and Author Addresses.
... say more about the use of alternative texts for images, tagging text spans and giving them an ID, and providing replacement texts for symbols and images
...
hyperlinks within the document, hyperlinks to external locations,
...
Where should hyperlinks to RFCs point? to the info page for the RFC? to the PDF version of the RFC? (NOTE: the RFC Series Editor has stated a preference for them to point to the info page for the RFC.)
...
Similar questions need to be answered on references to internet drafts: Where should hyperlinks to internet drafts point? To the datatracker entry? To the tools entry? To a PDF version of the internet draft?
...
a role-map should be provided here to map the logical tags found in the RFC XML to the standard tagset for PDF. This would be included in the generated PDF.
It has been suggested that the source input for code segments (e.g., ABNF, C code, MIBs) be extractable from the PDF. This capability might be supported through other mechanisms from the XML source files, but could also be supported within the PDF. PDF/A-2 (based on PDF release 1.7) allows for the embedding of some file formats, while PDF/A-3 adds support for arbitrary files to be embedded.
Another suggestion that has been made is that the XML input file itself could be embedded within the PDF. This would make the PDF file totally self-referential.
PDF has supported file signatures since PDF 1.2. It has been suggested that the PDF files be signed by the RFC Editor on creation. This would allow the signatures to be authenticated.
Recommendation: The RFC PDF documents created by the RFC Editor should be digitally signed.
Recommendation: Internet drafts do not need to be digitally signed.
NOTE: This section will talk about tools for creating, manipulating, transforming PDF files, including those currently in use by the RFC editor and Internet drafts, as well as outlining available PDF tools for various processes.
during either phase: generation.
during I-D phase: xml2rfc, of course for authors. Copy from PDF files? Review and comment. Digital Signature tools. Comparing two PDF files (versions).
During RFC publishing, xml2rdf. Editing of PDF to correct layout errors. Nits checking, checking conformance with PDF/a and PDF/ua.
There are profiles of PDF for specific purposes: PDF/UA and PDFA3 etc.
NOTE: add reasoning here about the recommendations.
Recommendation: use PDF/UA and also PDF/A3.
| [RFC3778] | Taft, E., Pravetz, J., Zilles, S. and L. Masinter, "The application/pdf Media Type", RFC 3778, May 2004. |
| [RFC6949] | Flanagan, H. and N. Brownlee, "RFC Series Format Requirements and Future Development", RFC 6949, May 2013. |
[RFC3778] contains some history of PDF. This is a capsule view, plus additional information on events that have occurred since the publication of [RFC3778]. NOTE: currently doesn't talk about the handoff of change control to ISO and the evolution as an ISO standard 32000. Plans are to update the application/pdf MIME registration to include this information, and then point to that.
The Portable Document Format (PDF) family of document formats was invented by Adobe Systems in the early 1990s. At the time, it was a proprietary format that underwent a variety of revisions that matched the release of different versions of the Adobe Acrobat products. For example, Acrobat 1 supported PDF version 1.0, Acrobat 2 supported PDF version 1.1, Acrobat 5 supported PDF version 1.4, etc. http://www.adobe.com/devnet/pdf/pdf_reference_archive.html
Each release (and extension level) introduced new features. For example, (1.0) character, word and image rendering, externally-referenced or embedded fonts, (1.1) passwords, encryption, device-independent color, (1.2) interactive forms, unicode, signatures, compression, (1.3) web semantic capture, embedded files, Adobe javascript, (1.4) metadata streams, tagged PDF, (1.5) controllable hiding of sections, slideshows, (1.6) 3D artwork, OpenType font embedding, linking into embedded files, and (1.7) video and audio support. After release 1.7, additional Extension Levels have been introduced. Each release also provided enhancements to the previous support. For example, encryption was introduced in 1.1, but AES encryption wasn't supported until 1.7 extension level 3. A PDF reader for PDF 1.1 is not able to read and display a PDF 1.7 file, but a PDF reader for PDF 1.7 can also handle all previous versions of PDF. The wikipedia page at http://en.wikipedia.org/wiki/PDF has a nice summary table going into further details.
Certain profiles or subsets of PDF have been standardized. PDF/X (X for Exchange), PDF/A (A for Archive), PDF/E (E for Engineering), PDF/VT (VT for Variables and Transactions), and PDF/UA (UA for Universal Access) all have ISO standards associated with them. Of particular potential interest to the RFC community are PDF/A and PDF/UA.
PDF/A in turn has nuances, as there have been a couple updates to it and conformance levels within each version. PDF/A-1 was based on PDF release 1.4. PDF/A-2 was based on PDF release 1.7, and PDF/A-3 adds embedded arbitrary files. PDF/A is considered a profile because it mandates that certain optional features be used. At a high level, the conformance levels are B (basic), U (mandatory unicode mapping [not in PDF/A-1]) and A (accessible). The requirements for conformance level A are that: the document structure must be represented within the PDF (e.g., section headings, table cells, paragraph divisions), tagged PDF is used (e.g., element anchors) and that language tags be used where appropriate. When referring to PDF/A, you would refer to the version and conformance level. So PDF/A-1A would be the profile for the Accessible conformance level of version 1 of PDF/A, which was based on PDF 1.4.
The PDF/UA (Universal Access) profile is orthogonal to the other profiles, specifying user accessibility requirements. It places some restrictions on the other profiles, such as requiring the use of higher-level constructs for the textual representation and adds additional requirements for programatic access (think automatic readers for the blind).
http://www.pdflib.com/fileadmin/pdflib/pdf/whitepaper/Whitepaper-Technical-Introduction-to-PDFA.pdf http://www.pdfa.org/wp-content/uploads/2011/08/tn0003_metadata_in_pdfa-1_2008-03-128.pdf http://www.pdfa.org/wp-content/uploads/2011/08/PDFA-in-a-Nutshell_1b.pdf http://www.pdfa.org/2011/08/pdfa-%E2%80%93-a-look-at-the-technical-side/ http://pdf.editme.com/pdfa