Am besten mit PHP + Headers:
http://ch.php.net/header z.B.
$n = "beliebigerDateiName.pdf";
$f = "effektiverDateiName.pdf";
header("Content-Disposition: attachment; filename=" . $n);
header("Content-Type: application/octet-stream");
header("Content-transfer-encoding: binary\n");
header("Content-Length: " . filesize($f));
header("Cache-Control: no-cache, must-revalidate");
header("Cache-control: private");
header("Pragma: no-cache");
header("Expires: 0");
$fp = fopen($f, "r");
print fread($fp, filesize($f));
fclose($fp);
exit();
Mit HTML + JS alleine geht das nicht.