First put the PDF into some random folder (I used my Downloads folder)
$> cp reading.pdf ~/Downloads
Then I started a simple HTTP Server there:
$> cd Downloads; python3 -m http.server
Then I used Linux's
ifconfig command to find my local network IP address.
Then I browsed to that IP on Safari, using the port that python3 was running the server out of. There was just a list of files in the directory that my iOS device could see, and I clicked on the filename of the PDF. Then I used the Safari upload button to transfer the PDF to the iOS Books app. The PDF is saved in iOS Books so that I can keep reading, so after that you can close down the http server on your Linux machine.