Human: What is the difference between a PDF file and an ePub file? Assistant: 
The main difference between a PDF file and an ePub file is that an ePub file is designed for reflowable content, meaning it can be read on various devices with different screen sizes. A PDF file, on the other hand, is more like a print version of your book that cannot be changed or resized to fit any device's screen size. An ePub file also allows users to add notes, highlights, and bookmarks, whereas a PDF does not.