I cannot find my PDFs on my iBook.... so many have been deleted.... how can I restore asap

I bought this new Apple iPad go back and look for a Download can you find them nowhere nothing is acting right one time they’re there next minute you’re gone what is the problem is there a malfunctioning what is Apple air

Hello,

Were these documents stored on iCloud? Is iCloud for Books disabled?

Settings > Books

I have books enabled for ICloud backup. When I went to ICloud there is no books, only photos, notes, email and downloads.