There are free utilities around which will let you crop the whitespace margins from a PDF. I have found
Briss to be pretty good, but there are others.
Calibre has one of its own (pdfmanipulate) but there is no GUI interface, you have to use the command line or write a small script. In both cases you would need to do this on the PC before sending the cropped PDF to your reader.
If you're going to read a lot of PDFs,
this article may be of interest.
I would have thought that the iPad is one of the better devices for reading PDFs.
Welcome to MobileRead, by the way