Within Microsoft Outlook, go to "File" menu and click "Options".
Select the "Trust Center" link.
Click on the "Trust Center Settings..." button.
Select the "Attachment Handling" menu.
Click on the "Attachment and Document Previewers..." button.
Locate the "PDF PreviewHandler" option from the "File Previewing Options" list, related to the program that you have installed that Outlook is currently using to preview file.
Right-click the PDF, choose Open With > Choose default program or another app in.
Select Always use this app to open .pdf files.