<?php

add_filter( 'woocommerce_email_attachments', 'my_child_attach_invoice_pdf', 10, 3 );

function my_child_attach_invoice_pdf( $attachments, $email_id, $object ) {
  if ( 'customer_processing_order' === $email_id && is_a( $object, 'WC_Order' ) ) {
    $invoices = wc_gzdp_get_invoices_by_order( $object, 'invoice' );
    if ( empty( $invoices ) ) {
      // Generate invoices
      $args = apply_filters( 'woocommerce_gzdp_invoices_defaults', array( 'invoice_status' => wc_gzdp_get_default_invoice_status() ) );
      $invoice = wc_gzdp_get_invoice( false, 'invoice' );
      $invoice->refresh( $args, $object );
      $invoice->mark_as_sent();
    }
    $invoices = wc_gzdp_get_invoices_by_order( $object, 'invoice' );
    foreach( $invoices as $invoice ) {
       $attachments[] = $invoice->get_pdf_path();
    }
  }
  return $attachments;
}