You will get more help if you display the problem here rather than in a link to another site
Some may not be happy clicking on an unknown link
Those links are fine. They are pointing to topics elsewhere which are discussing this very same issue in Manager.
Generally speaking, did PDF vendor confirm it’s not their fault and they blame PDF document itself? From those topics it’s not obvious.
Thanks Lubos, I have asked more specific question, here is their answer:
The method of font encoding used by the PDF writer (Apitron.PDF.Kit or a dependancy) does not include the glyphs mapped in a format understood by MuPDF Although it may be targeted in a way suitable for Adobe Reader.
Their typical font header is /Type/CMap/CMapName/Adobe-Identity-UCS/CIDSystemInfo <</Registry(Adobe)/Ordering(UCS)/Supplement 0>> /WMode 0/UseCMap/Adobe-Identity-UCS/
One solution/fix is to use another means to rewrite/reorder the font encoding such as GhostScript but the modified result whilst more universally readable, is still not perfect, possibly for the same reason.
Given a sample or two Artifex MuPDF @ Bugzilla (ghostscript.com) may (or may not) accept they can change MuPDF to make the translation work consistently.
I hope that helps manager to improve their PDF output. Regards.
There are a couple of other issues with the pdf within Manager. Could they be fixed at the same time?