The .ePUB file (from “electronic publication”) is an eBook in a widely used format.
There are two ePUB formats: Reflowable (with adjustable layout) and Fixed-Layout (with a fixed layout):

Reflowable: The text dynamically adjusts to the screen size or device and according to the reader’s settings (font size, alignment, background color).

Fixed-Layout: The page layout is fixed and remains the same on every device, similar to a PDF.