PDFs are exact representations of the page as the author intended, EPUBs are not. I generally find PDF far superior for nonfiction because it often has diagrams, pop out sections, etc. EPUB is fine for novels.
It's a tragedy that PDF to Epub conversion is generally not that good, I really wish more publishers (and selfpublishing authors!) would consider selling epubs as well.
At least I am yet to find one that can handle PDFs as easy as epub/mobi.
Somehow I suspect it is related with royalties to Adobe.
Reading standard screens is impossible in full daylight and also leads to sore eyes.