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