Environment
GroupWise 18
Situation
You want to preview in a GroupWise client a PDF attachment. However, when you click on the PDF file, preview fails.
Resolution
This might be related to older versions of Windows 10 or 7 with installed Acrobat Reader. When you face this problem, try to start Acrobat Reader DC application. Then go into Edit | Preferences menu option. Find there item Security (Enhanced). Here is by default enabled option "Enable Protected Mode at startup". Tick it off and close Adobe Reader.
Try to preview the same PDF attachment again.
In Windows 10 version 1903 and newer, this is not an issue despite this security settings in the Adobe Reader.
You can find out what Windows version you run by clicking on a search and type there winver.