That is mostly done by the MuPDF library, which SumatraPDF provides a nice interface for (along with supporting many other formats). I think the difficulty of

That is mostly done by the MuPDF library, which SumatraPDF provides a nice interface for (along with supporting many other formats). I think the difficulty of PDFs is not so much the spec, but the many many non-spec-compliant PDF files out there that need workarounds.