From some classes in university, I get a bunch of encrypted pdf files for each lecture. Its really annoying to type in the password of those files every time I want to open them, so here is a trick to remove the password protection from all files. You should have qpdf installed for this to work.
mkdir decr
for f in *.pdf; do qpdf --password=FOOBAR --decrypt $f decr/$f; done
Replace FOOBAR
by the password. All decrypted pdf files should now be in the folder decr
. If that worked, you can overwrite the encrypted pdf files and remove the folder afterwards:
mv decr/* .
rm -rf decr