No, there is no way set a default user name and password, but as an alternative you can set your policy to be "Anonymous Access", meaning that there is no authentication (of the user) required to open the document, but you can still control (i.e. prevent printing) what the "anonymous" user can do to\with the document.
Thanks, Steve. I noticed when I use anonymous access, the pdf will still prompt the user to confirm that anonymous data be sent to the server. Is there a way to avoid that prompt dialog?
I don't think that the dialog can be suppressed. If there is a "Don't show this dialog again" check box then the user can suppress it going forward, but they will always get the dialog the first time.