//fix from sarah

themes/default/order-detail.tpl

@@ -39,7 +39,7 @@
 {if $invoice AND $invoiceAllowed}
 <p>
 	<img src="{$img_dir}icon/pdf.gif" alt="" class="icon" />
-	<a href="{$link->getPageLink('pdf-invoice', true)}?id_order={$order->id|intval}{if $is_guest}&secure_key={$order->secure_key}{/if}">{l s='Download your invoice as a PDF file'}</a>
+	<a target="_blank" href="{$link->getPageLink('pdf-invoice', true)}?id_order={$order->id|intval}{if $is_guest}&secure_key={$order->secure_key}{/if}">{l s='Download your invoice as a PDF file'}</a>
 </p>
 {/if}
 {if $order->recyclable}