The minimum iOS version requirement for Acrobat Reader for iPhone and iPad version 19.02.01 or newer is iOS 11.2.
If you have an older iOS version on your iPad, you are not able to install the latest version of Acrobat Reader.
However, Apple App Store will allow you to download the last compatible version of any iOS app.
The trick is to use the Apple ID that was used to download and install the older version of the same app previously.
Hope this helps.