The substitution message exactly as reported by the conversion pipeline, verbatim and unparsed. For documents that expose font names structurally this may be null (use OriginalFontName / SubstituteFontName); for others it carries the full human-readable description, which names both the missing and the substitute font.
File name of the source document being converted. When the source was provided as a stream that is not a FileStream, this contains a generated identifier rather than a real file name.
Name of the font used as a substitute. May be null for documents whose engine reports the substitution only as descriptive text — in that case read Reason.