A 4.3-inch screen needs text that reflows
A PDF keeps the shape of a printed page. Shrunk to the Xteink's 4.3-inch screen, that page becomes a block of tiny text, even where the PDF opens at all.
An EPUB has no fixed page: the text wraps to the screen, so a book reads as comfortably on the Xteink as on a bigger e-reader, with fewer words on each page.
