The case of the 9 bit AES key

"MuPDF versions before 1.28.5 use an implementation of the drand48 family" yeah, ok, so they're basically not doing real encryption. The number of bits in the key space isn't the real story; the keys simply aren't cryptographically random.

Rule #1 of rolling your own RNG: don’t.

Rule #2? Please don’t.

Wonderful.

it's like the weak hardware wallet entropy bug

You'd have to be completely off your rocker to use a PDF tool written in C anyway.