Add billing and or shipping title to the address area. This code assumes that you have already included the necessary field on the checkout and have stored it in the order, you can use the WooCommerce Checkout Field Editor plugin for that. The title field should be called billing_title or shipping_title

gistfile1.txt

// Add a field to the customer's address, eg title
add_filter( 'woocommerce_order_formatted_billing_address', 'pdf_invoice_woocommerce_order_formatted_billing_address', 10, 2 ); 
function pdf_invoice_woocommerce_order_formatted_billing_address( $address, $order ) {

	// Add the post meta from the order
	$billing_title = get_post_meta( $order->get_id(), 'billing_title', TRUE );
	
	// Add $billing title to the beginning of the address 
	$address = array( "billing_title" => $billing_title ) + $address;

	// Return the address
	return $address;
	
}

add_filter('woocommerce_formatted_address_replacements', 'pdf_invoice_add_woocommerce_formatted_address_replacements', 10, 2);
function pdf_invoice_add_woocommerce_formatted_address_replacements($replace, $args) {

    $replace['{title}'] = $args['title'];
    $replace['{name}']  = $args['title']." ".$args['first_name']." ".$args['last_name']; //show title along with name

    return $replace;
}

add_filter( 'woocommerce_order_formatted_billing_address', 'pdf_invoice_update_formatted_billing_address', 10, 2);
function pdf_invoice_update_formatted_billing_address( $address, $order ){

	$address_fields = array(
		'title',
		'first_name',
		'last_name',
		'company',
		'address_1',
		'address_2',
		'city',
		'state',
		'postcode',
		'country'
	);

	if( is_array($address_fields) ) {

		foreach( $address_fields as $address_field ){
			$field = "get_billing_" . $address_field;

			if( method_exists( $order, $field ) ) {
				$address[$address_field] = $order->$field();
			} else {
				$address[$address_field] = $order->{'billing_'.$address_field};
			}
		}
	}

	return $address;
}

add_filter( 'woocommerce_order_formatted_shipping_address', 'pdf_invoice_update_formatted_shipping_address', 10, 2);
function pdf_invoice_update_formatted_shipping_address( $address, $order ){

	$address_fields = array(
		'title',
		'first_name',
		'last_name',
		'company',
		'address_1',
		'address_2',
		'city',
		'state',
		'postcode',
		'country'
	);

	if( is_array($address_fields) ) {

		foreach( $address_fields as $address_field ){

			$field = "get_shipping_" . $address_field;

			if( method_exists( $order, $field ) ) {
				$address[$address_field] = $order->$field();
			} else {
				$address[$address_field] = $order->{'shipping_'.$address_field};
			}
		}
	}

	return $address;
}