PDF files can be protected and locked using a password to disable it to be printed, modified or extracted.  Even when you know the password of a certain file, it is difficult to get rid of the password protection without removing the protection from it. 

There are two ways of doing it.

The first way is to print the files to a PDF printer like PrimoPDF.  But this works only if the files have permission to be printed.  Sometimes, saving the files with a different name also removes the password protection.

The second way, the easiest way is through a software; a free software.  Free PDF Unlocker removes all protection from your PDF files as its name say; for free.