PDF Invoice attachment to "order_conf" email is generated in the PaymentModule class.
The data for generated PDF Invoice is retrieved using the $order->getInvoicesCollection() method
i.e.:
public function getInvoicesCollection()
{
$orderInvoices = new PrestaShopCollection('OrderInvoice');
$orderInvoices->where('id_order', '=', $this->id);
return $orderInvoices;
}
And in PrestaShopCollection('OrderInvoice') there is definitely no custom data added by you to the customer's account.
In your module, you can add your own custom data via the displayPDFInvoice hook.