You can do this by adding the following code snippet to your theme functions. Read this first if you have never edited your theme functions: How to use filters.
add_action( 'wpo_wcpdf_after_order_details', 'wpo_wcpdf_coupons_used', 10, 2 );
function wpo_wcpdf_coupons_used ($template_type, $order) {
if( $used_coupons = $order->get_used_coupons() ) {
$coupons_list = implode(', ', $used_coupons);
echo '<h4>' . __('Coupons used') . '</h4>';
echo '<p>' . $coupons_list . '</p>';
}
}
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