How do I restore pdfs that are stored in the books app but not appearing on my iphone?

I recently set up an iPhone for my husband. In doing so I somehow managed to loose all of my documents and downloaded pdfs that were stored in the Books app on my

iPhone. I looked in my cloud there was nothing. So checked my storage, it shows the app and the size of its contents. But when I tap on it the page that opens asks me if I want to delete the files! I don’t want to delete them, I want to restore them. How would I go about restoring them?