Most of the blogs will offer you a solution to change the Browser File Handling property to Permissive in Central Administration for that particular web application.
First of all there are good reasons why SP2010 does not render files in browser, this change was made due to security reasons in SharePoint 2010. PDF included.
After considering the possible dangers you Could enable permissive mode, but that works for all file types. Instead it is much less of a security hole to just enable pdf as allowed inline mime types for the web application:
Hope this helps.......