>one of the most widely-used e-book formats
in the last 20 years has been the .pdf format --
a format which has not, historically, done reflow.

And which is a format that is universally recognized to be a page layout descriptor language, not an ebook file format.  PDF is a terrible thing to try to read on an ebook reader, unless the page layout happens to more-or-less match the size of your reader screen, and the size of the PDF font happens to be close to something your eyes like.  People tend to print PDF out if its more than a few pages because it is so much more suitable to a laser printer than to an ebook reader.  Google Books PDFs *do* happen to more-or-less often to match the size of the display on my DX and then its not too bad – although you are still reading a blurry photocopy with an occasional finger stuck in for good measure….

http://en.wikipedia.org/wiki/Portable_Document_Format