There are many ways of encrypting PDF documents. Adobe, in the PDF standard, originally developed a system of two levels of password encryption. In the first a password was used as the key to allowing you to edit, print, select text and graphics and copy them to the clipboard.
A second level was the user password, needed to open the document at all. In either case once you had opened the file there were no constraints of subsequent use of the content.
More sophisticated DRM systems do not rely upon passwords to provide decryption keys as the simple fact of having to disclose them, usually in uncontrolled email introduces a major weakness that DRM controls set out to prevent.
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