Hi Karlgunnars,
As shared that the sign in option isn't available at the upper right corner, that means neither the option nor your name showing at the top?
You must try installing the latest patch of the application through- Release Notes | Adobe Acrobat, Reader
Or uninstall the application using cleaner tool- Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs and reinstall it-
Adobe Acrobat Reader DC Install for all versions
Also, are you using this application on personal computer or is it a enterprise deployment managed by you IT administrator?
Note- If the application's installation managed by the IT Admin then there are chances that the "Sign In" of the application has been customized.
Regards,
Akanchha