Are you using a Mac? If so, what operating system are you running (10.4, 10.5)?
Also, Mac OS X can read PDFs without any 3rd party software using an application called Preview. Check to make sure that Preview is set as the default for reading PDFs. You can do this by right-clicking (or command + click) on the PDF document, clicking "Get Info", then expanding the "Opens With" section and selecting "Preview" from the drop down list.