PDF is not secure, therefore adding a password might stop a user only knowing Acrobat. However it is easy to overcome or even remove the password.
The main issue is that PDF security is supported by the reader, not the file format. I remember that some versions of Apple’s Preview didn’t respect security setzings, thus completely open.
The reason for that is in the PDF specs:
“PDF specifies a standard security handler that all viewer applications are expected to support, but applications may optionally substitute alternate security handlers of their own.”
Note: PDF cannot enforce the document access privileges specified in the encryption dictionary. It is up to the implementors of PDF viewer applications to respect the intent of the document creator by restricting access to an encrypted PDF file according to the passwords and permissions contained in the file."
So all you do is to hassle your client. They may want to print a lores, they may want to copy and paste text. And you are not protecting your artwork.
The suggestion I like best is to either rasterize the output or supply them just a bitmap (eg PNG). Or to trust your client.