Hi,
1. In the file plugins/hikashop/attachinvoice/attachinvoice/invoice.php add the HTML tag of your logo image:
<img src="http://mywebsite.com/my/image/file.jpg" />
2. Then use instead:
<div class="date">Order number: <?php echo $order->order_number; ?></div>
3. The language will be in the language of the customer. If you don't want that, you can remove the line:
$orderClass->loadLocale($order);
in the file plugins/hikashop/attachinvoice/attachinvoice.php and that will display the PDF in the current language when you click on the "invoice" button in the backend or frontend.