It's basically a css print file, which includes a lot of css properties that are not built in browsers yet, with a preview you can see on screen.

print is just another aspect of responsive web design. And paged.js add the possibility to add things that the paper need and the browser don't (page number, cross rereferences, running-head, etc.)

A bit more info about css print: https://www.smashingmagazine.com/2018/05/print-stylesheets-i...

We're sending webpages to printing press, and it becomes a book. That's it.