You need to send a header() directive to the browser that says "hey browser, here's a PDF. Do you know what to do with this? If not, can you ask the user what he/she wants to do with it? Thanks!" The browser then either opens it (if the browser has been set up to open PDFs, for example in the "Applications" tab in Firefox), or present a dialog box to the user along the lines of "Do you want to open or save this file?"
The header() directive looks something like this:
header('Content-Description: File Transfer');
header('Content-Disposition: attachment; filename='.$_GET['downloadfile']);
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Content-Length: ' . filesize($_GET['file']));