How to get a PDF in iPhone's Files app into the Books app?

PDFs that I Airdrop into my iPhone go to the Files app. I want certain ones in the Books app (search function works better, for one thing), and until recently it wasn't hard to accomplish but now I don't see a way to do it.

Originally, airdropped PDFs would just go into Books.

Then they started going into the Files app, but selecting a file and clicking the Share button would give a page with a strip of app icons across the top, one of which was Books.

Then Books was missing from that strip, but the More button at the end of it led to a vertical list of apps to move the file to, and Books was one of them.

Now Books isn't on the list, and Editing the list doesn't seem to allow adding apps to it, just turning on or off ones that are already there.

(clicking on the PDF in Files now opens a "Smart Printer" app that I don't want to have anything to do with.)

How can I get a PDF in my iphone's Files app into the Books app?