Before going in to my question, let me give you a little digression on my problem.
Some banks of mine no longer send me monthly balance sheets in paper by post, but only in PDF files for "environmental concern". Then I use a software to add comments to such PDF files as I had done so with paper balance sheets before. But some of these PDF files are locked with user password that I can't add any comment. I called my bank about this and the answer I got is to "print it out and add your comment"! Well... OK... that's very environmentally friendly...
So, right now I'm running pdfcrack to crack those PDF files using brut-force. Every file would take me more than two weeks to crack.
On the other hand and the main topic, I had tried to print out those locked PDF files to doPDF printer. But resultant PDF files have problems like some images don't show up. Is this an intentional behaviour precisely to prevent people from "converting" locked PDF files into unlocked PDF files?
OTOH, anyone of you has a better way or software to crack PDF files?