For a specific reason I have to be abble to unprotect a pdf file that I have previously crypted but I just can't code it ! How can I do that ?

For the moment I can open the encrypted pdf file, change the passwords (by setting them at null using "") and change the properties of encryptions (by enabling all rights on the pdf file). When it is done, I just save the file by when I'm opening it, I can always see the little key indicating that the file is encrypted.
Do you have an idea ?
thanks a lot.
NL