Is it a way to downgrade an app on my iPad downloaded from App Store?
I acknowledge this answer is based on a very literal interpretation of your question, but just to be thorough and in case it is useful to others looking for answers, here goes.

You could do this with a PC instead of a Mac, so long as you have past backups of the app in question on your PC or external backup drive.

To do this you need to download and install iTunes 12.6.3 (this can be installed alongside newer versions of iTunes). You would then need to copy the backup version of the iPad app that will work with your iPad to your iTunes Library. Then select and sync that version to your iPad from within iTunes.

Refer to Deploy apps in a business environment with iTunes on Apple's website for further information and download links.