Welcome @neilBar
Glad to have you here and thanks for your input. 
I’m late to the party, but one method no one mentioned (ideally for relatively short documents) is:
1.) Print the locked PDF (bonus: use a printer that can print on both sides of the paper)
2.) Scan the printed document and save it as an unlocked PDF
I wouldn’t try this method for a lengthy document/manual/book, but for “protected” PDFs that are fairly short in length, this technique has worked fairly well for me. Granted, it consumes a bit of paper and ink in the process, but sometimes that’s a price I’m willing to pay when the digital-only workflows require exponentially more time and effort to achieve a similar outcome.
In audio and video land this was referred to as the analog loophole or gap.
Get around all the copy protection by recording a signal over analog wires. The disappearence of
s-video and composite outputs on home video devices represents the closing of that gap. When everything has to go down a digital pipe, it is possible to control what can and can’t be done with it. Any opinions as to who controls, determines, or benefits from closing the analog gap are well…opinions.
It’s a practical solution in some cases. The irony here, which I quite enjoy, is that these files are scans of paper documents that existed pre-internet, so I do enjoy that a government employee scanned paper files and put them online for the public to use, and did it so badly that the best solution is to print them out again 
3 Likes