Lee Kung

sorry, the second point is already correct
here is what I do to get correct pdf_id

//in post loop
$image_id = get_post_meta( $post->ID, '_thumbnail_id', true );
if($image_id) {
    $image = get_post($image_id);
    if($image && $image->post_parent > 0) {
        $pdf = get_post($image->post_parent);
        if($pdf && $pdf->post_mime_type == 'application/pdf') {
            $pdf_url = wp_get_attachment_url($pdf->ID);
        }
    }
}