You can do this with Acrobat Standard or Acrobat Pro from File > Properties - Security.
You cannot do this with the free Adobe Reader.
Thanks. That's a great resource, but it won't do what I want to do. Firstly, it seems that the password on that screen if for permission to modify the document. I'm only looking to prevent unauthorized viewing. Secondly, the 3rd party document is often already password protected by its creator.
You can set a password to open the document, you can set a password to modify it, or both. But you can't do any of those things if it already has a password, without removing the existing security (which requires THAT password). A document can have only one set of security on it.