There are a few possibilities. The main PDF commands for text are "Tj" and "TJ", so you can try searching for them. Text commands must also be between "BT" and "ET". Maybe a bit harder to search for are the other two text commands: ' and ".
TJ is "Show text, allowing individual glyph positioning" which means strings are not necessarily in the PDF as the strings themselves, but have glyph positioning info mixed into them. This is the most common issue I've come across.
Another possibility is that the fonts have been combined/compressed/butchered, so that they're not using ASCII/Unicode (this can sometimes save space by combining bits of multiple fonts. Searching for Tj/TJ/'/" should tell you if that's the case. If it is, technically there should be a translation table somewhere in the PDF file (so that clipboard operations work in PDF viewers). Look for CustomEncoding, I think.
Particularly bad PDF creation libraries can put characters in one by one.
Characters (alone or in strings) can also be in there as octal or hex.
Finally, particularly obnoxious libraries can do it without text commands at all, but convert everything to vector drawing commands, or even worse, bitmaps.
I couldn't remember CustomEncoding, and this came up when I searched.
Seems helpful: https://stackoverflow.com/questions/29467539/encoding-of-pdf...
