Ben has article up right now ( link ) that explains the term "reflowable".   In it, Ben explains why the de facto standard for distributing documents has been PDF for a while, but may soon be switching to ePub for good reasons.   If you've ever tried to view a complex PDF file on your Kindle, you'll understand.
I fully agree with Ben on the value of ePub, and I think that the seemingly unassailable PDF is going to fall more quickly than anyone could have predicted.   My advice:  Get to know epub. 
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for people here and there, and it's what I've been recommending and pushing.