2potatocakes/pdf-reader — comment on commit 4fd1bcfa34d41cce2332c0e0c24d8b68c9fe276d
File: lib/pdf/reader/font.rb line 65
By yob:

unfortunately this isn't the best thing to do. Even if the raw bytes are all in the ASCII range (0-127) they may not be ASCII.

The PDF font has lots of ways to specify custom mappings of bytes to glyphs/characters.