The following 2 PDFs use some fonts that are not embedded (at least that's what Acrobat Preflight is telling me):
Running both files through Preflight I do see some font-related errors, but I don't see any occurrence of error 3.1.3, which is the specific error that I would expect here. Or perhaps this isn't this the unique error code for detecting non-embedded fonts?
Related side note: in "PreflightConstants.java" the description of 3.1.3 reads:
Error on the "Font File x" in the Font Descriptor
Which suggests it may be more generic than non-embedded fonts alone. But maybe this is not the case? (For myself I've only seen this error with an associated message like "Invalid Font definition, FontFile entry is missing from FontDescriptor for TimesNewRomanPSMT".)