I took a look at a few of these, and some of them are maybe just strange bugs.
E.g. the Adobe PDF spec embeds almost everything including most versions of Myriad, just not those two you mention. Similarly the Apple Guidelines embed Palatino Roman and Standard (as TrueType), just not Bold (as Type 1).
The Media Kit one does just not embed anything though (no Avant Garde), so that's clearly intentional.
It does make me wonder if technical documentation intended for a specific platform would sometimes try to save space by not embedding fonts standard not to PDF but to the platform. E.g. Avant Garde has shipped with Macs for a very long time. Still, what a terrible idea.
But fascinating to see documents with these problems in the wild, first time I've ever come across it. Thanks for taking the time!
It's probably not a coincidence that New Century Schoolbook was one of the fonts included with all PostScript Level 2 printers, but not included with older PostScript printers or Acrobat.
Note that the documentation in question was originally distributed as PostScript .ps files, not PDF, and it indeed makes no sense to embed New Century Schoolbook in a Level 2 or higher PostScript file, for both practical and likely legal reasons.
I took a look at a few of these, and some of them are maybe just strange bugs.
E.g. the Adobe PDF spec embeds almost everything including most versions of Myriad, just not those two you mention. Similarly the Apple Guidelines embed Palatino Roman and Standard (as TrueType), just not Bold (as Type 1).
The Media Kit one does just not embed anything though (no Avant Garde), so that's clearly intentional.
It does make me wonder if technical documentation intended for a specific platform would sometimes try to save space by not embedding fonts standard not to PDF but to the platform. E.g. Avant Garde has shipped with Macs for a very long time. Still, what a terrible idea.
But fascinating to see documents with these problems in the wild, first time I've ever come across it. Thanks for taking the time!