Using Outlook 2007 or 2010 you may encounter strange problem: PDF mail attachment won’t be opened with Acrobat Reader X (we monitored the problem with Outlook 2007 and 2010 using Acrobat Reader X 10.1.1).
While the attachment can be previewed in Outlook mail reader pane:

a double click on the same attachment in the same mail brings the Acrobat Reader X Window with error dialog box above saying: “There was an error opening this document. Access denied.”

The side effect is: if at least one Acrobat Reader X instance (program window on desktop) is already running, the attachment will be opened properly. Only if there’s no Acrobat Reader X window opened before you try to open a PDF mail attachment from Outlook – the error message appears.
The solution is:
- open Acrobat Reader X
- navigate menu Edit->Preferences:

- uncheck “Enable protected mode at startup”:

- confirm appearing dialog with “Yes”:

- close Preferences Dialog with “OK”
- close Acrobat Reader X
- retry to open PDF mail attachment
- …enjoy!