$path = "path/to/file.pdf";
$filename = "file.pdf";
header('Content-Transfer-Encoding: binary'); // For Gecko browsers mainly
header('Last-Modified: ' . gmdate('D, d M Y H:i:s', filemtime($path)) . ' GMT');
header('Accept-Ranges: bytes'); // For download resume
header('Content-Length: ' . filesize($path)); // File size
header('Content-Encoding: none');
header('Content-Type: application/pdf'); // Change this mime type if the file is not PDF
header('Content-Disposition: attachment; filename=' . $filename); // Make the browser display the Save As dialog