Historically, the former eBooks platform offered a mixture of PDF and ePUB books. The newer eBooks platform has gone to great lengths to convert all PDF books to a consistent ePUB3 format.
PDF-converted books will be represented in what is known as a fixed-layout ePUB format.
All native ePUB books are available in what is known as a reflowable ePUB format.
In a reflowable ePub, the content is fluid and fits the size of the screen. If you read on a smartphone, you’ll get properly sized characters, but will turn pages more often.
A fixed-layout ebook defines a viewport, i.e. the size of the “page” in pixels, and usually contains many images exactly positioned relative to text.