There doesn't seem to be an easy fix for this but it appears to be the same as it is in Acrobat. Unfortunately, you have to do this each time you want to clear all of the recent documents. On Windows 10, I open and run 'regedit'. Then go to HKEY_CURRENT_USER\SOFTWARE\Nitro\Pro\11.0\Recent File List. Then delete your recent documents. When you reopen Nitro Pro, your recent docs will be gone. Hope this helps.