Windows 10 defaults the opening of pdf files to its browser Microsoft Edge, to return it to Adobe Reader you need to do the following:
Ensure Adobe Reader is installed:
Follow the instructions from https://get.adobe.com/uk/reader/ to download and install Adobe Reader.
Set the default application for PDF files:
In Windows Explorer, right-mouse-click on a pdf file and go to: Open with > Choose another app
Then choose Adobe Reader as the application you want to use and make sure the “Always use this app to open .pdf files” is selected and click on OK.
Your pdf files will now open in Adobe Reader.