PDF is fine on portable devices
if the PDF's "pages" are sized to match the portable device they're used on.
That being said, fixed pagination, such as a PDF file offers, is a valuable characteristic when it's important for everyone to be "on the same page" as it were.
There are probably other ways to arrange that, but PDF is the only one that's currently widely available ... ubiquitous might be a better description.
On the other hand, prevalence alone doesn't make something "the best" for a given purpose. Air is pretty much everywhere, but it doesn't make a very good ... building material, for example.
