with the latest automatic update of Adobe Acrobat Reader it happened to us, on several PCs, that Windows 10 changed the default PDF viewer. This was not the case with the previous automatic updates of Adobe Acrobat Reader.
Hope you are doing well and sorry for the trouble. As described, Adobe Reader DC 64 bit turns itself into Adobe DC
Please go through the help https://helpx.adobe.com/acrobat/kb/about-acrobat-reader-dc-migration-to-64-bit.html and see if that helps.
The 64-bit application is a unified application for Acrobat DC and Acrobat Reader DC. The default installation location is C:\Program Files\Adobe\Acrobat DC. If you have installed Acrobat Reader DC 64-bit and you purchase an Acrobat DC subscription, the Acrobat Reader DC 64-bit application will upgrade to become a fully functional Acrobat DC 64-bit application.
Hope this information will help