Quickly Transfer a PDF from Linux to iOS without iCloud or AirDrop

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.