If you've ever run into this issue, it's a pretty easy fix.
First, see if a repair fixes the issue. To do this, go to Start, Control Panel, Add Remove Programs. Find Acrobat in your installed programs and select it. Then click on 'Change':
Click your way through this process by pressing 'Next'. Once that's finished see if you're able to view online PDFs.
If not, open Reader on its own and go to 'Edit'. 'Preferences':
On the left side, choose 'Internet':
Uncheck 'Allow fast web view' and go check out your online PDF!