Ваш HTML
<a href="downloader.php">Download pdf</a>
вы можете использовать с php save с downloader.php
$url = 'https://url-to-pdf-api.herokuapp.com/api/render?url=http://google.com';
set_time_limit(0);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
$r = curl_exec($ch);
curl_close($ch);
header('Expires: 0'); // no cache
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Last-Modified: ' . gmdate('D, d M Y H:i:s', time()) . ' GMT');
header('Cache-Control: private', false);
header('Content-Type: application/force-download');
header('Content-Disposition: attachment; filename="' . 'x.pdf' . '"');
header('Content-Transfer-Encoding: binary');
header('Content-Length: ' . strlen($r)); // provide file size
header('Connection: close');
echo $r;