There is a tool
K2pdfopt it's great and you can use it to optimize your PDF to your screen size.
Some custom readers have K2pdfopt library inside (
Koreader (for K5(Touch)/PW1/PW2+),
Librerator (for K2/K3/KDX and K4NT),
KPV (for K2/K3/KDX) I'm not sure about CoolReader3). It makes PDF readable on opening and works pretty good.
So you can either use K2pdfopt PC version to change your documents beforehand or use custom reader application. (but for that you'll need to JB your Kindle first)