Add billing field to Billing section, eg mobile number.

gistfile1.txt

// Add billing field to Billing section, eg mobile number. 
// Uses a placeholder [[PDFBILLINGMOBILE]] in the template file
add_filter( 'pdf_content_additional_content', 'custom_pdf_content_additional_content', 10, 2 );

function custom_pdf_content_additional_content( $content, $order_id ) {
	$content = str_replace(	'[[PDFBILLINGMOBILE]]', get_post_meta( $order_id,'mobile',TRUE ), $content );

	return $content;
}