PDF won't open with Adobe when attached to an email (Outlook)

I have the same problem. Brand new computer, latest Windows 10, latest Acrobat Reader. Someone said to remove Office 2016 (Office365) and reinstall it but I didn't do it. I wish these software companies would try out their software at home first instead of making us guinea pigs.

I have this exact problem using Acrobat Pro 2017 (classic).  Windows 10, Outlook 2016.  Outllok always asks "How do you want to open this file?"  The first option is "Keep Using" and acrobat is shown.  Other options are below, and at the bottom is a check box "Always use this app to open .pdf files".  I can check that box, but it still asks every time.  Once I click OK on this window, the PDF opens properly, I'm just trying to get rid of that box.  I have found dozens of users online who have complained about this issue, but no resolution anywhere.