If you force a user to save the file locally instead of opening it in the manner in which their browser is set up to handle it you're taking away their control of default behaviours. I really recommend against this. Let the browser handle it. If they have only the Acrobat Reader, a PDF will (generally) open within the browser. If they want to set it up differently to open in Acrobat itself, they can change their setup.
How many users out there do you know that actually know they can change these settings? Let alone know how to change these settings.
I have all these types of things set as I want them. I want to open Word and Excel files within the browser. If you force me to save it you're taking my preference away and I get really cranky. Then I have to remember where I saved it and clean it up later, rather than it being a temporary internet file that is purged automatically.
Forcing to save ALSO gives you the option to open, OK it does not open it in your browser, but in the program it was intended to open in the first place (outside your browser)..
I think no matter what preferences the user has set in their browser, we should do a bit of thinking for them, who actually has no problems opening up a PDF or word document in their browser?
my 2 pesetas