When a PDF is not allowing any form of editing, this is mostly because it is protected by a password by the creator. This is the reason why opening it can be done, but editing becomes impossible.
I've found Softaken Unlock PDF Files handy for this. With knowledge of the password, it unlocks the editing limitations and allows you to begin editing. It is effective in stripping away any constraints placed on multiple files simultaneously without altering the documents.
Tips: It is very important for you to keep a backup of your file prior to unlocking it because if something goes wrong when doing so, then all the information contained in the file might be lost. Once unlocked, the file can be opened and edited.
