If you just installed SP2010, you may notice that when you go to open a PDF file it prompts you to save it rather than opening
To change that, start sharepoint central administration and click ‘Manage Web Applications’
Scroll down the list until you reach ‘Browser File Handling’
Change the radio box from Strict to Permissive.
Click ok, and thats it.
Back to your documents library and try to open pd.
Good luck!