app/views/admin/document/invoices/show.html.erb
<h1> Invoice for Order(<%= @invoice.order.number %>) </h1>
<p>
<strong>Amount:</strong>
<%= @invoice.amount %>
</p>
<p>
<strong>Status:</strong>
<%= @invoice.state %>
</p>
<p>
<%= link_to "View PDF", admin_document_invoice_path(@invoice, :format => 'pdf'), :class => 'button' %>
</p>
<p>
<%= link_to "View All", admin_document_invoices_path, :class => 'button' %>
</p>