I figured it out:
I added this code
$var1 = wp_nonce_url( admin_url( 'admin-ajax.php?action=generate_print_content&template_type=invoice&order_id=' . $order->id ), 'generate_print_content' );
$var2 = esc_attr_e( '', 'woocommerce-delivery-notes' );
echo '<a href="'.$var1.'" class="button tips print-preview-button" target="_blank" data-tip="'.$var2.'"><span>PRINT INVOICE</span></a>';
right after the View button in my-orders.php line 88:
foreach( $actions as $key => $action ) {
echo '<a href="' . esc_url( $action['url'] ) . '" class="button ' . sanitize_html_class( $key ) . '">' . esc_html( $action['name'] ) . '</a>'; RIGHT HERE
After this, I commented the wp_die( __( 'You do not have sufficient permissions to access this page.' ) ); in class-wcdn-print.php line 68, 73, 78
