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As mentioned in other articles, it is possible to use passwords to protect PDF files from being opened, printed or altered.
In the Adobe PDF standard there are two passwords:
If the User password is already known, or the User password was not set, then there are literally hundreds of tools available, some free, some paid for, that will remove the Owner password almost immediately. (82 Million results on Google. Take your pick.)
So if you are trying to control document usage you have to set a User password on the document or the Owner password can be removed trivially.
This creates the obvious problem that the people you are sending protected documents to need to have the User password in order to read it. And once the user password is known users can then use PDF password removal tools to trivially remove the Owner passwords and do what they like with the documents. This is exactly the same weakness with sending encrypted documents that do not have DRM controls, the recipient can do what they like and you have no way of stopping them.
So the only time there is some control is if someone gets hold of a User password protected document, not knowing what the password is. But even here companies such as Elcomsoft, famous for being the first to break the owner password system, offer tools to carry out dictionary attacks (common password words such as ‘password’) and ‘brute force’ attacks (which will get you there eventually unless the password is very strong). Other tools may be found at Openwall.