iPad Downloads keep disappearing

I have several PDF files stored in Document Cloud and I want to access some of them on Acrobat Reader on several iPads, offline.

So, I locate the files, tap on the little cloud icon, the files download but after a few minutes or if I restart the iPad, the downloads disappear and I have to re-download them.

Some info about my setup:

- CC suite is being used on two Macs.

- The latest Acrobat Reader app has been downloaded on several up to date iPads, under 2-3 Apple IDs, and all are logged in and able to access the Document Cloud files.

- All the iPads have plenty of free storage.

- I have tried to remove Acrobat's access to iCloud drive, to no effect.

- The PDF files are about 10-20MB each

- I am trying to download about 30 of them for offline viewing

Any help?

P.S. What is the purpose of the "On this iPad" tab, if not displaying the downloaded PDF files?