The operating system requirement for the latest version (19.xx.xx) of Acrobat Reader for iPhone and iPad is iOS 11.2 or newer.
However, you can download the *last compatible version* of Acrobat Reader from the App Store.
The trick is to use the existing Apple ID that is associated with the older versions of Acrobat Reader to download and install the last compatible version from the "Purchased" section of the App Store app on an iPhone/iPad.
1. Go to iOS Settings.
2. Tap "iTunes & App Store".
3. Tap "Apple ID".
4. Tap "Sign Out".
5. Sign in with the Apple ID that was previously used to download the older version of Acrobat Reader.
6. Open the App Store app.
7. Tap your profile icon.
8. Tap Purchased.
9. Find Acrobat Reader in the purchased list.
10. Download Acrobat Reader.
Hope this helps.
It has been supported by even an android version 4.4.1, but not by an iOS 10.3.3!
Can't believe this!