How can I download Kindle books from Amazon on my iPad MD531LL/aA?

You have a first generation iPad Mini, which was released by Apple in 2012 and discontinued in 2015.

The latest version of iOS it can use is 9.3.5. (Current is 13.5.1.)

The Kindle app requires iOS 12 or later.

Your iPad is simply too old. You’re out of luck.

You have an original iPad Mini, which is locked at iOS 9.3.5 and cannot be updated past that. It is now too old, too under powered and a no longer supported device. The Kindle App requires iOS 12 or higher, which means you cannot download the Kindle App on such an old device. It's time for a more modern iPad as you'll find out most current apps are incapable of running on your aged device. Sorry