View the source of the pdf, find the trailer dictionary (ctrl-f "/Encrypt"), and note the object number of the encryption dictionary. An example trailer below,

View the source of the pdf, find the trailer dictionary (ctrl-f "/Encrypt"), and note the object number of the encryption dictionary. An example trailer below, here the encryption dictionary happens to be object ID 94,0 % Trailer dictionary trailer << /Size 95 % number of objects in the file /Root 93 0 R % the page tree is object ID (93,0) /Encrypt 94 0 R % the encryption dict is object ID (94,0) /ID [<1cf5...>] % an arbitrary file identifier >> Use the object id to find the encryption dictionary: % Encryption dictionary 94 0 obj << /Filter /Standard % use the standard security handler /V 1 % algorithm 1 /R 2 % revision 2 /U (xxx...xxx) % hashed user password (32 bytes) /O (xxx...xxx) % hashed owner password (32 bytes) /P 65472 % flags specifying the allowed operations >> endobj Generally, if /V and /R are both 4 or less, you can find tools to crack it yourself on a normal PC. More info on the values of /V and /R here: https://qpdf.readthedocs.io/en/stable/encryption.html https://qpdf.readthedocs.io/en/stable/encryption.html