I have no affiliation with the author and this is the first I'm seeing the content, but I'd guess its's because PDFs can be vector-scalable and never lose their quality as you scale up/down and PNGs cannot.
Sure you can. But it isn't nearly as certain to generate the same kind of output when changing printers; platforms or programs. PDF at least does that part well and I totally support the authors choice and think .png would be a terrible format for this.
They're meant to be posters, pdf is entirely appropriate.
