PDF standards comparison: PDF/A vs PDF 1.4
The PDF file format, far from being a unique language definition, gathers an extensive family of sublanguages (which we friendly call dialects) and specifications. These specifications make the format richer, and allow a better fitting between the language restrictions and requirements of real scenarios. On the other hand, the library has to consider all these sublanguages and specifications to correctly read and write PDF files.
Thus, these specifications must be studied, compared and formalised, in order to allow the GNU PDF Library provide an API capable of processing syntax and semantics accordingly.
This study is contained in the GNU PDF Knowledge base, available here. Until now, only the comparison between PDF/A and PDF 1.4 is available, but more sublanguages of the PDF family are coming in the near future. Stay tunned!