If you ever find it necessary to remove a password from your encrypted PDF file without modifying the document in anyway, then there is a tool for that. The tiny app or utility that solves the problem is called BeCyPDFMetaEdit. Its a free Windows utility that you can also install on your Linux box, be it Ubuntu or Mint or whatever via WineHq.
Here’s how to get along:
First, launch the program. Before you select and open the PDF, change the mode to “Complete Rewrite.” Lastly switch to the Security tab and set the “Security System” to “No encryption.” Click the Save button and your PDF will no longer require a password to open.