rdmit

Hi Ewout, yes you are right. I have actually figured out what to do and this snippet worked in my case.

// Display the custom field inside the PDF Invoice and Packing slips

add_filter( 'wpo_wcpdf_after_order_data', 'rdm_po_number', 10, 2 );
function rdm_po_number( $template_type, $order ) {
	if ( $template_type == 'invoice' || $template_type == 'packing-slip' ) {
		$rdm_po_number= $order->get_meta('Purchase Order Number');
		echo "<tr style='padding:0.5rem; background: #ededed;'><td>Your PO no.:</td><td>{$rdm_po_number}</td></tr>";
	}
}

That was the final part of the snippet, the full one is as follow and works fine for me:

/ 1 Let’s add a new field to checkout, after the order notes, by hooking into the following:

/**
 * Add the field to the checkout
 */
add_action( 'woocommerce_after_order_notes', 'rdm_custom_checkout_field' );

function rdm_custom_checkout_field( $checkout ) {

    echo '<div id="rdm_custom_checkout_field"><h2>' . __('Your Purchase Order no.') . '</h2>';

    woocommerce_form_field( 'rdm_po_number_field', array(
        'type'          => 'text',
        'class'         => array('rdm-field-class form-row-wide'),
        'label'         => __('Add your internal purchase order number'),
        'placeholder'   => __('XX-123354'),
        ), $checkout->get_value( 'rdm_po_number_field' ));

    echo '</div>';

}

// 2 Next we need to validate the field when the checkout form is posted. For this example the field is required and not optional:

/**
 * Process the checkout
 */
add_action('woocommerce_checkout_process', 'rdm_custom_checkout_field_process');

function rdm_custom_checkout_field_process() {
    // Check if set, if its not set add an error.
    if ( ! $_POST['rdm_po_number_field'] )
        wc_add_notice( __( 'Please add your internal purchase order number.' ), 'error' );
}

//3 Finally, let’s save the new field to order custom fields using the following code:

/**
 * Update the order meta with field value
 */
add_action( 'woocommerce_checkout_update_order_meta', 'rdm_custom_checkout_field_update_order_meta' );

function rdm_custom_checkout_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['rdm_po_number_field'] ) ) {
        update_post_meta( $order_id, 'Purchase Order Number', sanitize_text_field( $_POST['rdm_po_number_field'] ) );
    }
}

// 4 If you wish to display the custom field value on the admin order edition page, you can add this code:

/**
 * Display field value on the order edit page
 */
add_action( 'woocommerce_admin_order_data_after_billing_address', 'rdm_custom_checkout_field_display_admin_order_meta', 10, 1 );

function rdm_custom_checkout_field_display_admin_order_meta($order){
    echo '<div style="padding:0.5rem; background: #D46A6A; border: 1px solid #670d0d " role="alert"><strong>'.__('Purchase Order Number').':</strong> ' . get_post_meta( $order->get_id(), 'Purchase Order Number', true ) . '</div>';
}

// Addd custom fields to the email 
/* To use: 
1. Add this snippet to your theme's functions.php file
2. Change the meta key names in the snippet
3. Create a custom field in the order post - e.g. key = "Tracking Code" value = abcdefg
4. When next updating the status, or during any other event which emails the user, they will see this field in their email
*/
add_filter('woocommerce_email_order_meta_keys', 'rdm_custom_order_meta_keys');

function rdm_custom_order_meta_keys( $keys ) {
     $keys[] = 'Purchase Order Number'; // This will look for a custom field called 'Tracking Code' and add it to emails
     return $keys;
}
// Display the custom field inside the PDF Invoice and Packing slips

add_filter( 'wpo_wcpdf_after_order_data', 'rdm_po_number', 10, 2 );
function rdm_po_number( $template_type, $order ) {
	if ( $template_type == 'invoice' || $template_type == 'packing-slip' ) {
		$rdm_po_number= $order->get_meta('Purchase Order Number');
		echo "<tr style='padding:0.5rem; background: #ededed;'><td>Your PO no.:</td><td>{$rdm_po_number}</td></tr>";
	}
}