A PDF is a marvelous invention. When you prepare documents, there is no point in sending them thru doc files or just text files to people who can easily modify it. Particularly when you need to get a document signed, you simply cannot use a word file. You cannot edit the contents of the PDF even with a PDF editor without the password.

The PDF is still an open document that can be read by anyone with any freely available pdf reader. You could use a PDF DRM software If you have high value pdf documents to protect and therefore require stronger protection than simple passwords then look no further than Lizard Safeguard PDF Security from LockLizard. It prevents copying,
saving and sharing of your PDF documents, prevents or limits the
number of prints and views, and prevents screen shots from being
taken. You can expire content after a certain date or instantly
revoke access to it on demand. Best of all, there are no passwords to enter or for users to pass on to others and the system is protected with US Government strength security.