Mom’s iPad will download the books but then the cover image will disappear and she can no longer access them

I have been using z library myself for a while now on my google books and chrome tablet. I got to wiki pages- then browser to download them onto my google books. My mom has an iPad and I tried downloading Avery Flynn books as epubs on Apple Books. They stopped working for her, so I tried pdfs but she needs to zoom in the font (bad eyesight) so those don’t work for her. Is z library not feasible with Apple products? Any ideas on how to solve the problem? I kept trying to look for different versions of the same books, but they all end up not working.