Can’t save books to files app

I’ve found if I download an epub file or PDF and save it directly to the files app I can then click on it in the files app and it will open in the books app. However, if I open the downloaded file directly with the books app, I don’t see a way to save the document to the files app. I’ve tried editing and sharing but the option to send the document to the files app doesn’t seem to exist. Can anyone help with this?