Set Outlook Default PDF Previewer

Solution 1:

• 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 files.
• When you disable the checkbox, that viewer will no longer be in use:
• Reinstall the software that you want to be the actual viewer/previewer/handler and restart Outlook

Solution 2:

In Windows 10, search for "Default apps"

From the Default apps screen, scroll all the way to the bottom

Click on Choose default apps by file type

Scroll down the file extension list to .pdf

Choose the app you want to open the .pdf file from Outlook

I am not sure how you pick an app that's not on the list though. Perhaps that's a different question. For me, I just needed to go from Microsoft Edge to Google Chrome.