Online converter

https://ebook.online-convert.com/convert-to-epub

https://www.zamzar.com/convert/epub-to-pdf/

https://calibre-ebook.com/ — (https://lifehacker.com/5509965/how-can-i-convert-pdfs-and-other-ebooks-to-the-epub-format)

Using Adobe InDesign to convert PDF to ePub: https://forums.adobe.com/thread/976831

and

https://www.quora.com/What-is-the-best-free-PDF-to-EPUB-converter

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more on ePub in this IMS blog
https://blog.stcloudstate.edu/ims?s=epub