Hi Gary,
I have seen this issue occur with users who have this registry setting missing or have improper permissions.
When you double click the attachment to launch the pdf it is saved and opened from the temporary folder unless saved specifically to a location.
If you dont have the below mentioned registry key or improper permissions then attachments fail to open.
Please heck if the following reg key exists:
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Security
or
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\14.0\Outlook\Security
Value Name: OutlookSecureTempFolder
If the value exists, and if the value contains a valid path, Outlook 2010 uses that location for its temporary files.
If the registry value does not exist, or if the value points to an invalid location, Outlook 2010, Outlook 2007, or Outlook 2003 creates a new subdirectory under the Temporary Internet Files directory and then puts the temporary file in the new subdirectory. The name of the new subdirectory is unknown and is randomly generated, depending on your version of Outlook.
Please check the following key:
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Security
Note the location for: OutlookSecureTempFolder key
Check if the same folder exists in the following location and if not create a corresponding folder in the location on C:\Users\<User account name>\Appdata\Local\Software\Microsoft\Windows\......
Regards,
Rave