You have set Foxit as default PDF viewer on your system (How to set Foxit as Default PDF Reader), but running into the following problem with Outlook Previewer:
The Outlook Previewer is being reset to something else. The icon is still Foxit, but the preview window is not.
The Outlook Previewer window cannot preview any PDF attachments.
The above problem mostly occurred after a system update, office update or software reinstall.
To reset outlook previewer to Foxit, complete one of the following methods.
Set Foxit previewer handler as default by modifying registry entry
Follow the instructions mentioned in this article to set Foxit previewer handler as default via modifying registry entry.
Re-register the previewer
Open Task Manager, look into the Processes tab, see if there is any “FoxitPrevhost.exe” running there, if any, right click on it and choose End Task
Follow here to re-register Foxit previewer plugin “FoxitPrevHndlr.dll”..
After done, reload Outlook and try the Previewer.
Reinstall the application is the most effective way to restore all default settings with only three steps:
Uninstall. Remove the existing Foxit Application by going to Control Panel > Programs and Features (Uninstall or Change a Program).
Download and install. If you have the latest installer file on your machine, find it and install it, If don’t, access the download center: https://www.foxitsoftware.com/downloads, download the correct version and execute it to install.
Activation. Use your original registration code to activate the program. If you are a subscriber of the application, sign in using your subscription account to confirm and activate it.
Note: in the process of the installation, ensure “Set Foxit as the default PDF viewer” is checked in the Setup screen.