Code to add border to the invoice pdf - By WebToffee ( WooCommerce PDF Invoice, Packing Slips, Delivery Notes and Shipping labels )

functions.php

<?php //Do not copy this line of code
add_filter('wf_pklist_add_custom_css','wt_pklist_alter_border_style',10,2);
function wt_pklist_alter_border_style($custom_css,$template_type)
{
	if( 'invoice' === $template_type )
	{
		$custom_css.='body{ margin:30px !important; border:1px solid #000; }';
	}
	return $custom_css;
}