I've asked about this before:
https://www.kinook.com/Forum/showthread.php?t=5481... still having problems.
Trying to use a non-Adobe pdf viewer within UR. My latest attempt is to use Sumatra. In Windows' Default Settings I have set pdf files to be opened with Sumatra.In Windows Explorer I have likewise set .pdf files to be opened with Sumatra. For good measure I also uninstalled Adobe Reader DC.
And indeed PDFs are now opened with Sumatra, however UR will not display the PDF in the browser, instead it asks if I want to open or save. Choosing "Open" opens the PDF in Sumatra, not in UR.
Is it impossible to open a PDF within UR using anything other than Adobe Acrobat? If it is then I will give up trying...