Given the program could hugely benefit (I suppose) from current
multi-core/multi-process computers, it would be beneficial to use version which
is not single-threaded.

There are at least two forks of pdfcrack which support multithreading:
https://github.com/MichaelSasser/pdfcrack-ng (Latest commit c66b973 on 15 Jun
2016) and https://github.com/shreepads/pdfcrack-mp (Latest commit 7b12017 on 15
Nov 2015, but with multiple forks the most developed seems to be
https://github.com/irate-badger/pdfcrack-mp with the last commit on
2019-02-26).

Hmm, I can see the reasons, why you don't want to be involved with such mess.