lopdf does not output xref table or trailer dictionary as per spec

[fschutt]

1. The trailer dictionary is missing the required trailer keyword, i.e. lopdf outputs << /Root 5 0 R/Size 6 ... >> instead of trailer<< /Root 5 0 R/Size 6 ... >>
2. lopdf doesn't output any standard plain-text xref table? In every newly PDF generated by lopdf, there is no xref table, only an /XRef stream on the trailer dictionary.

This leads to errors in some PDF viewers, see mozilla/pdf.js#19007

[Heinenen]

There are two ways in the spec to write the cross-reference information, the "old" way as an xref table and the "new" (since PDF1.5) way as a cross-reference stream.

The default way that lopdf does it is via the cross-reference stream. The stream does not require the trailer keyword (cf. 7.5.8 Cross-reference streams).

The value following the startxref keyword shall be the byte offset of the cross-reference stream rather
than the xref keyword. For PDF files that use cross-reference streams entirely ([..]), the keywords xref and trailer shall no longer be used.

If you prefer the classical xref table over cross-reference streams, you can configure lopdf to output those instead by changing the xref type:

let mut doc = Document::with_version("1.7");
doc.reference_table.cross_reference_type = XrefType::CrossReferenceTable;

I couldn't reproduce the issue by myself. Could you maybe post some way to reproduce your issue?

PS: The PDF in your linked issue is not spec compliant because it is only PDF1.3 which are not allowed to have cross-reference streams (only since PDF1.5). If you don't have the specific requirement to use PDF1.3 I highly recommend switching to at least PDF1.7. By now, PDF1.7 is 16 years old, so all readers should be able to handle it.

[fschutt]

Thank you. It's not really about the age of the PDF standard, it's just that some PDF standards are set for a specific conformance and for print proofing, so Acrobat will spit out errors when sending the PDF file to a printing company.