In my experience, pdf should be first converted to HTML file using
mobipocket creator. This HtML can be built into a PRC file by the same software. The results are pretty decent going thru this route.
If you have a multi column pdf, then I remember I saw a software here at the forums that could convert those pdfs to single column ones. Then, you know the drill, convert to HTML, build it to PRC (which is nothing but a mobi file with a different extension and is perfectly compatible with K3).