Add a test PDF that uses custom page numbers

This was downloaded from
https://www.w3.org/WAI/WCAG20/Techniques/working-examples/PDF17/page-numbers.pdf,
found via https://www.w3.org/TR/WCAG20-TECHS/PDF17.html.

dev-server/templates/index.mustache

@@ -71,6 +71,7 @@
     <li><a href="/pdf/painting">PDF without selectable text</a></li>
     <li><a href="/pdf/gatsby">The Great Gatsby</a> (A long document)</li>
     <li><a href="/pdf/jstor">PDF with JSTOR banner</a></li>
+    <li><a href="/pdf/custom-page-numbers">PDF with custom page numbers</a></li>
   </ul>
 
   <h2>Page feature tests</h2>