Auto saving in Android, Acrobat Reader

Actually autosave is not an automatic on Android.  You are given an option when filling out any type of information. Especially if it's repeatingly. This is given as an option due to case sensitive information. I personally would highly recommend not filling out important information with autosave/ auto fill. Not only for security reasons but also business and personal apps are usually not autosave friendly. Good luck!

Auto saving in mobile apps is instant, and it has to be because iOS/Android systems will suddenly kill any app, any time, without warning and with no chance for the app to save. So the idea of "Save" is essentially obsolete on mobile.

Yes. You should have an on your keyboard.  Go to settings and then Keyboard.  There should be an option to turn off auto fill. On some it is on the keyboard where you see a setting icon. Hit that and you should also be given an option there as well.  Good luck!