You can edit the .ascx file to make the invoices open in the same windows/tab. To do this, open MyAccountManageOrders.ascx in the Smith.MyAccount module, and find(Lines 68-71):
<ItemTemplate>
<asp:HyperLink ID="hlInvoice" runat="server" NavigateUrl='<%#GetEditUrl(DataBinder.Eval(Container.DataItem,"OrderID")) %>'
Target="_blank"><%= Localization.GetString("Invoice.Text", LocalResourceFile)%></asp:HyperLink>
</ItemTemplate>
Make the following changes, Highlighted in yellow this to (Change Target="_Blank" to Target="_Self"):
<ItemTemplate>
<asp:HyperLink ID="hlInvoice" runat="server" NavigateUrl='<%#GetEditUrl(DataBinder.Eval(Container.DataItem,"OrderID")) %>'
Target="_self"><%= Localization.GetString("Invoice.Text", LocalResourceFile)%></asp:HyperLink>
</ItemTemplate>
Thanks,
Christian