maybe this helps:
If you want the user to be prompted to save the data you are
sending, such as a generated PDF file, you can use the Content-
Disposition header to supply a recommended filename and force
the browser to display the save dialog.
<?php
// We'll be outputting a PDF
header("Content-type: application/pdf");
// It will be called downloaded.pdf
header("Content-Disposition: attachment; filename=downloaded.pdf");
// The PDF source is in original.pdf
readfile('original.pdf');
?>
Note: There is a bug in Microsoft Internet
Explorer 4.01 that prevents this from working. There is no
workaround. There is also a bug in Microsoft Internet Explorer 5.5
that interferes with this, which can be resolved by upgrading to
Service Pack 2 or later.