You'll get a lot of people telling you PDF is a horrible format to try and convert to an e-book, and they're mostly right, but...
If you want the PDF to fit better on your kindle screen you might try
k2pdfopt. It is not always great with classroom-style text books, but it's worth a try. There are videos that show you how to get started in the
help pages.
If that doesn't work for you, try my
PDF conversion tips page.