Is there a way to create a pdf so that it automatically logs in with a default username and password so that the user is not prompted upon opening?
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.
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.