Several popular programs frequently set themselves as the default application for opening PDF files in Windows 10, even without the user's knowledge. The most common examples are Microsoft Edge and Google Chrome. You can set the program for opening PDF files the way you prefer.
Find any PDF file on your system.
Right-click the PDF file and select "Open with".
Don't select the program here. Instead, click on "Choose another app".
Select the program that you would like to use for opening PDF files. Most Shield-managed systems are configured for Nitro Reader or Nitro Pro, but you can choose what you want.
Check the box marked "Always use this app to open .pdf files" and click "OK" to save.
Now, when you open PDF files, they will open in the program you had selected.