Unless I'm on a paper-sized tablet I would definitely rather have an offline HTML file than a PDF. Nobody likes to pan back and forth on lines of text to read something.
I was skeptical at first, but I think the author made the point fantastically well.
Your browser has a zoom functionality that lets you make the text smaller, essentially replicating the PDF site above. Only the opposite of what you say is correct: I can’t read that PDF’s text without turning my phone into landscape and picking up my glasses.
I get what they're going for but the PDF is not exactly an accessible reading experience.
(EPUB is basically a subset of HTML with client-oriented context.)
There’s a reason responsive design has been a big deal for the last 10+ years and I don’t think the benefits of PDF are worth throwing it out.