Acrobat Reader can often be a source of annoyance, and one challenge is getting it to work on 64-bit systems -- but you can get the job done with a simple registry fix.
After finding that the Reader plug-in for previewing documents in Outlook wouldn't work on his 64-bit system, Lifehacker reader Jon wrote in pointing us to a registry hack that fixes the problem. Hit the link for full details, including a ZIP file which automates the process of changing the registry.
As ever, registry changes should be made with caution, and this isn't a recommended (or necessary) step for 64-bit users. To simplify your PDF-viewing life, check out the Hive Five of the best PDF readers. Thanks Jon!