I looked at the source code for that page and except for an HTML file header and footer it is entirely javascript, which your browser interprets/renders into an image that is expressed in PDF for display on the screen. When you print the file there is no printable image because there is no interpreter for the javascript, but when you choose Print to PDF, it grabs the PDF display image the browser, or any other application, generated to create the screen display and sends that to the printer.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein