PDF is excellent for printing, but for reading on-screen, it sucks. :)
For what it's worth, I personally think PDF is a pretty decent format for
viewing this kind of material. My own user experience with HTML
documentation has been mostly bad; I've found HTML to be great for reference
material, where there's a natural way to chop up content into small,
relatively independent bits, but bad for more in-depth narrative material.
The problem I've had with in-depth material in HTML is that you seem to get
either very long pages, where scrollbar navigation becomes a huge pain, or
you get material that's chopped into so many small pages that page
navigation becomes a huge pain. I blame the browser more than the format;
there's no reason in principle that a web browser couldn't do a better job
with very long pages, but it's certainly not what present browsers are
optimized for.