PDF is meant to be a digital equivalent to paper, thus it does not scale well to different screen sizes. An objective of PDF is for a document to look the exact same no matter where you open it.
ePub text and images can be reflowed to fit a smaller/larger screen, it is not proprietary, file sizes are smaller, and is overall an ideal format for ebooks.
ePub text and images can be reflowed to fit a smaller/larger screen, it is not proprietary, file sizes are smaller, and is overall an ideal format for ebooks.