The obvious way would be to edit the margins of the original document.
I don't know about Kingsoft, I use Cute PDF myself. It allows scaling in the advanced options. Print -> Choose printer -> Options on the right of the printer drop down menu -> Advanced. Just tested: It appears to center the text horizontally no matter what the scaling is, starting from top of the document, though. I tested with a 500% scale and the center words/letters on the top rows were centered. On a two page document it cut all the excess from the bottom away instead of adding pages. Anyway, with the right scaling percentage it should work like you want.
Try to find similar settings in the applications you use.