You have gotten me curious about this now.

Certainly creating a pdf would be easier but EPUB is becoming the industry-standard file format for reflowable digital publications.
EPUB files requires you to set up your Indesign file differently.


How is an EPUB eBook different from a PDF eBook?

The EPUB format is an XML-based format designed to enable text to reflow according to the capabilities of various eBook readers, which means that you can resize the text, change the font, or view an eBook on different screen sizes, and the text will reflow to fill the available view area. This makes the EPUB format the best choice for eBooks that are read on small handheld reading devices.

In contrast, PDF preserves the original layout of a document, giving you complete control over page design and presentation. PDF is the optimal choice for eBooks that have a complex design or will only be read on regular-sized computer screens.
Mix, try google search “how to create epub ebooks”
Maybe you can find some step by step tutorials.

Share with us what you have learned.