the actual paper content format should be separated from its rendering.
i.e. it should contain abstract, sections, equations, figures, citations etc. but it shouldn't have font sizes, layout etc.
the viewer platforms then should be able to style the content differently.
I disagree. PDF is the most desirable format for printed media and its analogues. Any time I plan to seriously entertain a paper from Arxiv, I print it out first. I prefer to have the author's original intent in hand. Arbitrary page breaks and layout shifts that are a result of my specific hardware/software configuration are not desirable to me in this context of use.