The current minimum iOS requirement is iOS 11.2. If you have an older iOS version, you will not be able to install the latest version of Acrobat Reader.

Here are a couple of options that you can try.

(a) Update iOS on your iPad/iPhone

If you have a relatively new iPad/iPhone (e.g. iPhone 5s/SE or later OR iPad Air 1st gen. or later), you should be able to update to iOS 12.1. After updating iOS to the latest version, you can now install the latest version of Acrobat Reader.

(b) Download the last compatible (not the latest) version of Acrobat Reader

When you go to the App Store app on your iPad/iPhone and try to install the latest version of Acrobat Reader, do you see the alert with the title “Download an older version of this app?” and the Download option?

If so, you can choose the Download option from the alert to download and install the last compatible version of Acrobat Reader.

© Use other Apple ID (that is associated with older versions of Acrobat Reader) to sign into App Store.

If the App Store app did not provide an option to download the last compatible version of Acrobat Reader, you can sign out from iOS Settings > iTunes & App Store, tap your Apple ID, sign out, and sign in with the Apple ID that was previously used to download and install older versions of Acrobat Reader.

Hope this information helps.