You would like to track a pdf file download as a goal in Google Analytics. Accomplish this task using virtual pageviews.
1. Modify your original download link with the _trackPageview method. For example:
<a href="http://www.example.com/report.pdf" onClick= "pageTracker._trackPageview ('/Reports/Nasa.pdf');">Download a PDF</a>
2. Within an appropriate profile create a new Goal.
3. Use the following configuration for your new Goal. Note that the Goal URL in the following screen shot matches the virtual pageview that you created in step 1.