If you just click on the link in the post, it will try to use the Acrobat Reader plug-in for your browser, not the standalone program that you installed. If your browser hasn't been configured with the Adobe Reader plug-in, it won't display. If you right-click on the link and actually download it to your computer, you'll be able to view it in the Acrobat Reader application.

For help with browser-related problems with Adobe Reader, you can check Adobe's support page.