[WP] Auto open fancybox for first PDF attachment

<?php // Show attachments for the current post
$args = array(
    'post_type' => 'attachment',
    'numberposts' => -1,
    'post_status' => null,
    'post_mime_type' => 'application/pdf',
    'post_parent' => $post->ID
    );

$attachments = get_posts($args);

if ($attachments) {

    foreach ( $attachments as $attachment ) {

        // build embed link
        $embed_link = 'http://docs.google.com/viewer?url=' . wp_get_attachment_url( $attachment->ID ) . '&amp;embedded=true';

        // thumbnail
        $output = '<a href="' . $embed_link . '" class="fancybox-iframe fancybox" id="fancybox-auto">';
            $output .= get_the_post_thumbnail( $post->ID, 'large' );
        $output .= '</a>';

        // print
        echo $output;
    }

}