PMPro PDF Invoices add PDF Invoice attachment to "Membership Expiring" emails

pmpro-pdf-invoices-membership-expiring.php

<?php
/**
 * Include PDF in the membership expiring email.
 * This will get the member's last successful order and attach it.
 * Add this code to your site by following this guide - https://yoohooplugins.com/customize-wordpress/
 */
add_filter( 'pmpropdf_pdf_included_email_templates', function( $templates ) {
    // Add membership_expiring to the list of templates that get PDF invoices
    $templates[] = 'membership_expiring';
    
    return $templates;
} );