Within Microsoft Outlook, go to "File" > "Options".
Select "Trust Center"
Click on "Trust Center Settings..."
Select "Attachment Handling"
Click on "Attachment and Document Previewers..."
Locate the "PDF Preview Handler" option from the "File Previewing Options" list, related to the program that you have installed that Outlook is currently using to preview files (often times if Adobe was the previous app, it will be listed as the Adobe preview handler).
When you disable the checkbox, that viewer will no longer be in use:
Reinstall the Foxit software that you want to be the actual viewer/previewer/handler and restart Outlook