> Because very few 'end user' people know how to use en/decryption
That's the point, rather than PDF viewers implementing this, it should be an OS-wide UX, so that you could click any encrypted file and have a unified password prompt, regardless of whether it's a PDF, PNG, DOCX, etc.
And the same goes the other way, rather than having your PDF viewer implement the encryption UX, you should just have an "Encrypt with password" entry in your file manager's context menu.
That's the point, rather than PDF viewers implementing this, it should be an OS-wide UX, so that you could click any encrypted file and have a unified password prompt, regardless of whether it's a PDF, PNG, DOCX, etc.
And the same goes the other way, rather than having your PDF viewer implement the encryption UX, you should just have an "Encrypt with password" entry in your file manager's context menu.