Some time ago i noticed that when I opened a PDF file, Adobe Reader X stopped responding for sometime. I noticed it happens on Windows 2003, Windows 7 and Windows 2008. after some investigation I found a solution in disabling Adobe’s “new” Protected Mode feature.

  1. Open Adobe Reader X
  2. navigate to Edit->Preferences->General
  3. clear the check-mark at “Enable Protected Mode at startup”
  4. Click OK to save the changes (acknowledge the warning)
  5. 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)