Yes you can, and "It Depends". For best result run them through the KindlePDF Converter/Optimiser, which is somewhere on these boards. If I can find the link I'll come back and post it
*Edit*
Here you are:-
http://willus.com/k2pdfopt/ . Drag your PDF onto the .exe file and hit enter to see how it looks. Sometimes you need to do a bit of manual tweaking to the parameters first.