Existe até um exemplo disso em php.net
<?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');
?>
Ou expanda um pouco com
<?php
if ( can_this_file_be_downloaded() ) {
header('Content-type: application/pdf');
header('Content-Disposition: attachment; filename="invoice.pdf"');
readfile("{$_GET['filename']}.pdf");
} else {
die("None shall pass");
}
?>