Windows 10 will sometimes change the default PDF viewer / writer program from Nitro Pro to Microsoft Edge (e.g. when it installs monthly Windows Updates). Here's how to reset Nitro as the default PDF:

1. Click on Windows Start button and select Settings.
2. In the Windows Settings screen click on Apps.
3. On the left-hand side pane of the Apps & Features screen click on Default apps.
4. On the Default apps screen go to the right-hand side pane and scroll down then click on Choose default by file type.
5. In the Choose default apps by file type screen scroll down until you find under the Name column .pdf PDF Document. If the program listed next to .pdf is not Nitro Pro (e.g. Microsoft Edge or Chrome) then click on it.
6. In the Choose an app screen: click on Nitro Pro.