While font formats have become more modern and overall fonts have become higher quality, problems with fonts are still a sizeable chunk of what goes wrong with PDF documents.
At VIP Event 2017, Dietrich von Seggern (Managing Director of callas) started the session with some font theory and provided an overview of the tools offered by pdfToolbox to deal with font problems.
02:06 - Font tools in pdfToolbox
02:38 - Default profiles: List & Fix potential font problems
06:15 - 'Glyph', '.notdef glyph' and 'empty glyph'
08:58 - How are fonts used in PDF page description
14:16 - How does PDF page description refer to glyphs
20:17 - Font types
23:17 - True Type fonts
28:55 - Mapping to Unicode
32:55 - Some more definitions for technical font related terms
37:12 - 'List potential font problems' Checks and their relation to PDF/X-4 and PDF/A-2
39:12 - What can go wrong? practical examples
39:20 - Missing glyphs
43:17 - .notdef glyphs is not a notdef
43:51 - Glyph substitution
47:22 - Font tools in pdfToolbox in more detail