If a Mac user is unable to preview a PDF attachment on their emails, and they have to save it in order to view the file this is usually a bug within Microsoft Outlook. Usually, you get an error that says The Operation Failed The File cannot be previewed because of an error with the following Previewer, ‘PDF Preview Handler
If you download Adobe Reader this usually fixes the issue but Mac users would rather use the Mac PDF reader so the only solution is to update Outlook.
To Update Outlook on a Mac
Open Outlook and go to Help – Check for Updates
The new release of Outlook 365 has fixed this issue
