Acrobat Reader DC not opening, but prompting Acrobat DC to open

++Adding to the discussion,

I think you may be confusing that  configuirng the web browser's preferences to   open PDF's in its own browser tabs is acomplished via plugin.

From the way your post is worded it could be understood as if you were trying to default your PDF's accessed through your default Internet browser to be opened directly with Adobe Reader DC or Acrobat Pro DC, for example.

You can set your browser preferences to use the Adobe Reader Plugin instead as shown here step by step: https://helpx.adobe.com/acrobat/kb/pdf-browser-plugin-configuration.html

But if you wish to always open a PDF document that is accessed through an URL in your default web browser you have to set in your broswer's "Preferences--->>>Downloads"  to always ask where to save the file.  And THEN, in your operating system default the PDF ownership to be handlded by  the  Reader DC or Acrobat Pro DC. 

You also didn't clarified if when you say that is requiring you to log in it actually means that you have to login with your AdobeID to be able to access documents that are stored in a document cloud service OR to unlock the premium editing tools and features of Acrobat Pro DC (in the case where you have a Trial version, or , recently subscribed to a plan and haven't activated it yet).

Why do you have both Acrobat and Reader installed at the same time? Also, what versions are they, and what's your OS version?