Acrobat on iPad prompts me to sign in to my existing OneDrive account and does not present me with a keyboard to type the email or password. This makes Acrobat useless to me as I have tons of PDF's stored there.
I hope you are doing well. I am sorry for your experience with Acrobat Reader Mobile.
I tried to reproduce the process at my end and could sign in to a OneDrive account without issues.
It looks more like a gltich from iOS than Acrobat Reader Mobile. I suggest you restart the device once and then check if you are able to fill in the details.