i had a similar issue with our printable PDFs recently (which print from a custom-built invoice generator, not ubercart)

my advice to fix it is to get your hands on both *firebug* and chris pederick's *web developer extension* for firefox

install a print-only CSS file for your theme so you can customise print settings

then use web dev toolbar => CSS => by media type => print to view (in the browser) the page as it will print

and then use firebug to customise/explore from there - you can tweak the CSS live with that - i'd try right-clicking in the white space at bottom of page and use "inspect element" to see which element occupies that space

possibly your body or html tags have a large margin-bottom or padding-bottom which is forcing the extra page

keep copying changes into the print.css, hitting print to PDF and seeing if the results work

good luck!