What's a good alternative, for users and developers?

I don't have any love for PDF, but I'm actually not sure what's more cross-platform. Any browser will render PDF, so everyone already has a viewer on their computer. A browser will also print any document to PDF, and many other editors can export to PDF (though perhaps not import for editing)

It can't be replaced by an Office format, like docx, because even today apps like Pages can't render MS Office docs correctly half the time.

Doesn't seem like HTML would fly, either, given all the kinds of things that get embedded into PDF.