I realize this is an old post, and my comment is a special case, but I thought it was worth mentioning this:
Yes, a typical PDF, with its fixed formatting, is usually designed for a page larger than a Kindle screen, and is therefore usually way too small to read comfortably. However, it's worth noting that — should the project call for it — a PDF can look very polished and readable on a Kindle if it is designed for the Kindle.
As an example, one recent project called for a set of cartoons with both captions and a few lines of commentary under each one. Rather than risk the horrendous layout mess that would occur if the reader happened to be using a slightly too-large font, we designed a Kindle-specific InDesign document with pages 600 x 800 px, and produced a PDF that looks exactly the way the client wants it on the Kindle. Although the reader can't resize (and reflow) anything, the result is a very clean and controlled layout with none of the usual typographic Kindle issues.
There are other similar situations where the frozen formatting of a PDF is desirable, and designing a custom document that is readable on a Kindle is worth the effort. One additional advantage (again, only in certain situations) is that you can get the PDF onto a specific Kindle (e.g., your client's Kindle) by simply emailing it to the target Kindle's owner's account on Amazon.
So although PDF isn't the format of choice for eBooks in general, it can be an excellent solution in special situations, and shouldn't be ruled out across the board.
Allen
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