EPUB, open PDF alternative supported by the largest number of hardware readers (en.wikipedia.org)

It's not an alternative to PDF. It's an alternative to Kindle's MOBI. PDF is intended for printing, and ePUB with its XHTML and CSS has nothing that makes printing easy.

By the largest number of brands, sure (everything except Kindle?)

But certainly not by the largest number of devices. Kindle dwarfs everything.