There appears to be Adobe’s “new” Protected Mode feature.
- Open Adobe Reader X
- navigate to Edit->Preferences->General
- clear the check-mark at “Enable Protected Mode at startup”
- Click OK to save the changes (acknowledge the warning)
- Shutdown Adobe Reader X
You can also disable protected mode through a registry key
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Acrobat Reader\10.0\FeatureLockDown]
“bProtectedMode”=dword:00000000
EDIT: In Acrobat Reader XI the location hast changed to: Edit->Preferences->Security (Enhanced)
Now that might be it...
taken from
SOLVED: Adobe Reader X hangs after opening a PDF file - van Efferen
(there might be smarter people arround...)