Add rel="nofollow" to only PDF links in Wordpress content and excerpt

gistfile1.txt

add_filter('the_content', 'my_nofollow');
add_filter('the_excerpt', 'my_nofollow');
function my_nofollow($content) {
    return preg_replace_callback('/<a[^>]+\\.pdf/', 'my_nofollow_callback', $content);
}
function my_nofollow_callback($matches) {
    $link = $matches[0];
    $link = preg_replace("%(href=\S(?!$link))%i", 'rel="nofollow" $1', $link);
    return $link;
}