In using PDF & Print, I have made two changes to the links for SEO sake. I put a nofollow on the print & pdf buttons and also changed the page_id to page-id (for some reason, SEO wants dashes in URLs).
Something for the next update...
$permalink = add_query_arg( 'print' , 'pdf' , get_permalink() . '?page-id=' . $post->ID );
$str .= '<img src="' . plugins_url( 'images/pdf.png', __FILE__ ) . '" alt="image_pdf" title="View PDF" />';