FindTrueHeaderPosition: Determine header offset to locate XREF entries.

[DW715]

Third-party tools can write blocks before the header block of the PDF, this causes validation to fail as the version number cannot be read. FindTrueHeaderPosition() determines the header offset whose value is used to locate XREF entries.

[Numpsy]

Can the 'stuff' that gets written before the PDF Version header contain '%' (or text which looks like the PDF marer but isn't?)

[DW715]

Can the 'stuff' that gets written before the PDF Version header contain '%' (or text which looks like the PDF marer but isn't?)

I suppose it might, if it did I don't think you could ever find the 'real' header without validating the whole XREF table for every possible match?

[Numpsy]

The PdfPig equivalent (https://github.com/UglyToad/PdfPig/blob/8a82500427ace6d4dcb1b2ea7cb3fdb5e32c765d/src/UglyToad.PdfPig/Parser/FileStructure/FileHeaderParser.cs#L94) appears to be searching for the whole %PDF- bit and then checking for that being followed by a version number - is it possible to do at least part of that or is it too big of a change here?

[DW715]

is it possible to do at least part of that or is it too big of a change here?

I'll have a look, was trying to keep all changes to a minimum.

[DW715]

@Numpsy done.