convert e-pub to different format

I am using a public library book app called BorrowBox and have downloaded an e-book to my PC using Adobe digital editions. The aim was to transfer the file to my e-reader. I want to use my local library rather than buying books all the time. Unfortunately my Kindle paper white e-reader does not accept e-pub format so I am trying to convert the book from e-pub to either pdf or text or doc.  It may be read only or password protected but any site I use to convert says it can't do it. Any ideas how I can read the book on my kindle? It apparently works on the kindle fire but that is no use to me.