He (probably) means that the geometry of a PDF "page" can be customized. You can even have different sizes within the same document. But most people using LaTeX or even basic plotting utilities which export to PDF know this.

A properly prepared PDF will scale "perfectly" (FSVO - needs all drawings in vector form) as long as you keep the aspect ratio the same.

The problem is changes in aspect ratio or wanting to reflow text to have larger font when rendering to small screen.

Not sure if it's doable to generate flexible enough rendering commands in PDF. Was doable in PostScript (again, FSVO) but PDF was explicitly made to be decidable and less turing complete.