When I install the latest version i can no longer open any of my older PDFs It just hangs on opening and does nothing for several minutes until I finally just close the window.
This is on a windows 7 64 bit machine. My only solution is uninstall the 11 and reinstall the older 9 which will open the files just fine.
Please try to disable the Protected Mode and see if that works.
Edit-> Preferences-> Security Enhanced and uncheck the enable the Protected Mode at Startup and Enable Security Enhanced checkbox. see if that works.
Also, please follow the steps mentioned here: http://helpx.adobe.com/acrobat/kb/cant-open-pdf.html
However, there is no entry for Privileged at HKCU\Software\Adobe\AcrobatReader\11.0 - in fact there is no entry betweeen ~\Originals and ~\RememberedViews
I've tried the Repair option in Control Panel\All Control Panel Items\Programs and Features, but with no change to the problem behaviour. My system is 64-bit Windows 7.
I notice that pdf attachments will preview correctly in Outlook 2010.