It works, but that doesn't mean it's good. A document designed to be printed on an A4 page isn't easy to read when scaled down to the size of a phone, especially for people with suboptimal eyesight, and if you zoom in to read the text, you need to scroll horizontally for every line you want to read
I'm aware of all that, although your complaint about zooming in is easily fixable by turning the phone sideways. My point is that PDF remains popular because it's predictable and because print layout standards are generally better than dynamic ones. That's the problem to address: people's desire to read longform text in a consistent way without being continually derailed by interactive cruft.
