PDF is very important for the academic world, being the document format of choice for most journal publishers. Not everyone is happy about that, partly because reading page-oriented PDF documents on screen (especially that expletive-deleted double-column layout) can be a nightmare, but also because PDF documents can be a bit of a semantic desert. Yes, you can include links in modern PDFs, and yes, you can include some document or section metadata. But tagging the human-readable text with machine-readable elements remains difficult.
In XHTML there are various ways to do this, including microformats (see Wikipedia). For example, you can use the hcard microformat to encode identifying contact information about a person (confusingly hcard is based on the vcard standard). However, there are relatively few microformats agreed. For example, last time I checked, development of the hcite microformat for encoding citations appeared to be progressing rather slowly, and still some way from agreement.
The alternative, more general approach seems to be to use RDF; this is potentially much more useful for the wide range of vocabularies needed in scholarly documents. RDFa is a mechanism for including RDF in XHTML documents (W3C, 2008).
RDF has advantages in that it is semantically rich, precise, susceptible to reasoning, but syntax-free (or perhaps, realisable with a range of notations, cf N3 vs RDFa vs RDF/XML). With RDF you can distinguish “He” (the chemical element) from “he” (the pronoun), and associate the former with its standard identifier, chemical properties etc. For the citation example, the CLADDIER project made suggestions and gave examples, for example of encoding a citation in RDF (Matthews, Portwin, Jones, & Lawrence, 2007).
PDF can include XMP metadata, which is XML-encoded and based on RDF (Adobe, 2005). Job done? Unfortunately not yet, as far as I can see. XMP applies to metadata at the document or major component level. I don’t think it can easily apply to fine-grained elements of the text in the way I’ve been suggesting (in fact the specification says “In general, XMP is not designed to be used with very fine-grained subcomponents, such as words or characters”). Nevertheless, it does show that Adobe is sympathetic towards RDF.
Can we add RDF tagging associated with arbitrary strings in a PDF document in any other ways? It looks like the right place would be in PDF annotations; this is where links are encoded, along with other options like text callouts. I wonder if it is possible simply to insert some arbitrary RDF in a text annotation? This could look pretty ugly, but I think annotations can be set as hidden, and there may be an alternate text representation possible. It might be possible to devise an appropriate convention for a RDF annotation, or use the extensions/plugin mechanism that PDF allows. A disadvantage of this is that PDF/A (ISO, 2005) disallows extensions to PDF as defined in PDF Reference (Adobe, 2007), but PDF/A is important for long-term archiving (ie that such extensions are not compatible with long-term archiving). I don’t know whether we could persuade Adobe to add this to a later version of the standard. If something like this became useful and successful, time would be on our side!
What RDF syntax or notation should be used? To be honest, I have no idea; I would assume that something compatible with what’s used in XMP would be appropriate; at least the tools that create the PDF should be capable of handling it. However, this is less help in deciding than one might expect, as the XMP specification says “Any valid RDF shorthand may be used”. Nevertheless, in XMP RDF is embedded in XML, which would make both RDF/XML and RDFa possibilities.
So, we have a potential place to encode RDF, now we need a way to get it into the PDF, and then ways to process it when the PDF is read by tools rather than humans (ie text mining tools). In Chemistry, there are beginning to be options for the encoding. We assume that people do NOT author in PDF; they write using Word or OpenOffice (or perhaps LaTeX, but that’s another story).
Of relevance here is the ICE-TheOREM work between Peter Murray-Rust’s group at Cambridge, and Pete Sefton’s group at USQ; this approach is based on either MS Word or OpenOffice for the authors (of theses, in that particular project), and produces XHTML or PDF, so it looks like a good place to start. Peter MR is also beginning to talk about the Chem4Word project they have had with Microsoft, “an Add-In for Word2007 which provides semantic and ontological authoring for chemistry”. And the ChemSpider folk have ChemMantis, a “document markup system for Chemistry-related documents”. In each of these cases, the authors must have some method of indicating their semantic intentions, but in each case, that is the point of the tools. So there’s at least one field where some base semantic generation tools exist that could be extended.
PDFBox seems to be a common tool for processing PDFs once created; I know too little about it to know if it could easily be extended to handle RDF embedded in this way.
So I have two questions. First, is this bonkers? I’ve had some wrong ideas in this area before (eg I thought for a while that Tagged PDF might be a way to achieve this). My second question is: anyone interested in a rapid innovation project under the current JISC call, to prototype RDF in PDF files via annotations?
Adobe. (2005). XMP Specification. San Jose.
Adobe. (2007). PDF Reference and related Documentation.
ISO. (2005). ISO 19005-1:2005 Document management — Electronic document file format for long-term preservation — Part 1: Use of PDF 1.4 (PDF/A-1).
Matthews, B., Portwin, K., Jones, C., & Lawrence, B. (2007). CLADDIER Project Report III: Recommendations for Data/Publication Linkage: STFC, Rutherford Appleton Laboratory.
W3C. (2008). RDFa Primer: Bridging the Human and Data Webs. Retrieved 6 April, 2009, from http://www.w3.org/TR/xhtml-rdfa-primer/