A PDF preserves the exact layout of the printed page — fonts, spacing, and design remain fixed regardless of screen size. It is best suited for larger screens such as laptops or tablets.
An EPUB is a reflowable format: the text adjusts and reflows to fit the screen size and font preferences of the reader. EPUBs are ideal for e-readers such as Kindle or Kobo, and for smartphones, offering a more comfortable reading experience on smaller screens.