That's because you're trying to print from your browser but the pdf isn't really there. Your browser opens a blank window and then overlays it with a separate program (possibly Adobe Reader) which displays the pdf. If you use the browser's 'print' option, it will only see the blank window.
You need to select the print option from within Adobe Reader (or whatever program is relevant here), rather than the print option from your browser.
At this point though I've got a problem in explaining exactly what to do, as the different possible combinations of browsers and pdf readers means that what you see on your screen probably isn't the same as what appears on mine. All I can say is that you need to look for a 'Print' button (which might well be a button with a printer logo on it, rather than the actual word 'print') somewhere within the window which is open in your browser, rather than using the browser's print option.