When I try to update version 20.11.00 in the Apple App Store, it says:

Download an older version of this app?
The current version requires iOS 13.3 or later but
you can download the last compatible version.

Below this message there are two buttons: Cancel and Download

When I press Download, it just redisplays the message above. Maybe
whatever indicates which app version is compatible with which iOS
version is messed up. Or maybe the Acrobat version I have (20.11.00)
is the last compatible version for my version of iOS and I should not
have gotten a message asking me to update the app. I am using iOS
12.3.1 and I am not able to upgrade the iOS version.