First of all, the file signature is %PDF-n.m (8 bytes). After that the docs says there might be at least 4 bytes of binary data (but there also might not be any). The docs don't say how many binary bytes there could be, so that is my first question. If I was trying to parse a PDF file, how should I parse that part? How would I know how many binary bytes (if any) where placed in there? Where should I stop parsing?
Which docs do you have? The PDF specification ISO 32000-1 says:
If a PDF file contains binary data, as most do (see 7.2, "Lexical Conventions"), the header line shall be
immediately followed by a comment line containing at least four binary characters—that is, characters whose
codes are 128 or greater.
Thus, those at least 4 bytes of binary data are not immediately following the file signature without any structure but they are on a comment line! This implies that they are
- preceded by a
% (which starts a comment, i.e. data you have to ignore while parsing anyways) and
- followed by an end-of-line, i.e. CR, LF, or CR LF.
So it is easy to recognize while parsing. In particular it merely is a special case of a comment line and nothing to treat specially.
(sigh, I just saw you and @Jongware cleared that in comments while I wrote this...)
What could be the minimal file size of a PDF, assuming there isn't anything at all (no objects, whatsoever) in the PDF file and assuming the file doesn't contain the optional binary bytes section at the beginning?
If there are no objects, you don't have a PDF file as certain objects are required in a PDF file, in particular the catalog. So do you mean a minimal valid PDF file?
As you commented you indeed mean a minimal valid PDF.
Please have a look at the question What is the smallest possible valid PDF? on stackoverflow, there are some attempts to create minimal PDFs adhering more or less strictly to the specification. Reading e.g. @plinth's answer you will see stuff that is not PDF anymore but still accepted by Adobe Reader.
Third and last question: If there were more than one body+xref+trailer sections, where would be offset just before the %%EOF be pointing to?
Normally it would be the last cross reference table/stream as the usual use case is
- you start with a PDF which has but one cross reference section;
- you append an incremental update with a cross reference section pointing to the original as previous, and the new offset before
%%EOF points to that new cross reference;
- you append yet another incremental update with a cross reference section pointing to the cross references from the first update as previous, and the new offset before
%%EOF points to that newest cross reference;
- etc...
The exception is the case of linearized documents in which the offset before the %%EOF points to the initial cross references which in turn point to the section at the end of the file as previous. For details cf. Annex F of ISO 32000-1.
And as you can of course apply incremental updates to a linearized document, you can have mixed forms.
In general it is best for a parser to be able to parse any order of partial cross references. And don't forget, there are not only cross reference sections but also alternatively cross reference streams.
