What is an EPUB file?
EPUB is an ebook format whose text adapts to your screen. PDF keeps fixed pages. Use EPUB for reading and PDF for printing.
How EPUB works
An EPUB is really a zip file of web pages, pictures and styles. Your reader app lays out the pages for your screen and font size.
EPUB or PDF?
For long reading on a small screen, EPUB wins. For anything where layout matters, such as forms, printing and design, PDF is better.
What about DRM?
Many store-bought ebooks are encrypted so only approved apps can open them. Those files can't be converted.