Microsoft Edge hijacking default PDF handler in Outlook 2019

I assume that by Acrobat DC not Pro, you're referring to Adobe Acrobat Reader DV (free version PDF viewer).

If you go to the apps settings in Windows 10, you can select the default PDF handler from these settings (a restart may not be required but it never hurts)...

Is this the defaulting method that you've tried?

Another way, is to go to the folder where your PDFs are saved, right-click on any of them, select "Properties" from the context menu and change the program from MS Edge to Acrobat Reader... click OK and close the dialogue window to commit the changes.

Also, you may want to do the same in your MS Edge browser, and disable from the current settings the option to open PDFs in a new browsing tab and instead choose to download PDF when you click on URLs that contain a PDF.

That said, you may also try and install the Acrobat PDF Maker add-in (or extension) from the Chrome webstore for your MS Edge browser.

Even though that extension is designed primarily to take advantage of online PDF editing services(with a paid subscription), it integrates well with Acrobat Reader.

Nevertheless, it may be a good idea to uninstall the Acrobat Reader completely and use the cleaner tool before you reinstall or perform any of the methods mentioned above.