In Acrobat Reader:
Go To Edit>Preferences
Under Categories, Select Internet, then Uncheck "Display PDF in Browser"
If that doesn't work, try
Go to Tools>Options
Select "Downloads", Click on Plug-ins (bottom right above Cancel)
Scroll until you find the file type PDF, then uncheck enabled.
Note, I use Firefox 1.0.7 and Acrobat Reader 7.0, I have my browser setup so that it downloads PDF instead of displaying them...