This is a known issue with the Edge browser in Windows 10. To reset the default program for opening .pdf files, perform the following steps.
1. Search on 'control' and go into Control Panel.
2. Go into Default Programs > associate a file type or protocol with a program.
3. Scroll down until you find .pdf and select it.
4. Click on the Change program button in the upper right-hand portion of the window.
5. Click on Adobe Acrobat Reader DC, then click on Ok. Exit this window and Control Panel.