How to Recover Lost Downloads on iPad?

I’ve been using an IPad for school the past year. I would download files from each class of my program to take notes on them. I’ve been keeping them in my files as I’ll need them to prepare for boards I’ll have to take. I noticed today that my IPad somehow has removed several of my saved downloads from my files. I couldn’t find them anywhere, not even in the recently deleted. I’m not sure how long they’ve been gone but I know I had them downloaded and somehow they’ve just disappeared.

[Re-Titled by Moderator]