You need to download any content to your browser to view it; otoh, online and in-browser renderers for both epub (which is mostly web tech in a package format) and PDF are ludicrously common.
> Superbook format on the other hand is a website. Huge difference!
Superbooks, on a quick glance through some, seem to have a visual look much like epub, the wasted space from aspect ratio mismatch of PDF, and the speed/responsiveness of cold molasses.
> PDFs are a skeuomorphic parallel of formatting that's natural to physical paper where as Superbooks adopt narrow column formatting of pure web with styling that scales responsively.
I don't see how PDF is any more skeumorphic than Superbooks (certainly, most PDF readers don't use skeumorphic—and dog slow—page turn animations.) You can, of course, design PDF with any fixed layout you want.
And AFAICT the main responsiveness Superbooks have is automatically choosing between single-page and two-page view based on screen aspect ratio/orientation.
I don't know what the “narrow column formatting” referred to is; the only webbish formatting I see is that pages seem to have symmetric bordering white space on left and right pages (but still have left and right pages distinguished, as in the version of 1984, by page number location.)
