With PDFs the Kindle doesn't change the font size; all it can do is change the zoom (magnification) factor which is why you only see part of the page the other part should show up if you use the thumbstick. (I think; I avoid PDFs like the plague.)
If it is a text-only PDF, you could just turn it into a Mobipocket file and then it woud behave like a real ebook file.
If you go to:
http://www.mobipocket.com/en/downloa...ilscreator.asp
you can download the MobiCreator app that will automatically convert pdfs into Kindle-compatible file (for free--Amazon charges something like 10 cents to do the same thing).
Just make sure you install it in the Pro version not the personal.
Then just import the pdf and copy over the prc/mobi file to the Kindle.