if all you have is a pdf, it very likely will not convert nicely to epub without some work on your end. if you’re just trying to view a textual document, like a novel or something, the big advantage of an epub is that it’ll reflow to fit your screen. you can change the font size or style, margins, line spacing, whatever, and its like the book was just printed that way, and you can flip pages naturally. if your pdf has a small font, you’ve got to scroll and zoom and it is a big pain in the butt. pdfs are designed for printing first and foremost, while epubs are meant to be read digitally.