test backend attribute value in PDF version

Author: OlivierJourdan

www/en/modules/ROOT/pages/index.adoc

@@ -18,4 +18,6 @@ NOTE: This page is the entry point of the course.
 
 ifeval::["{backend}" != "pdf"]
 * {site-url}/{page-component-name}/{page-component-version}/{page-component-name}.pdf[PDF version of the course]
-endif::[]
+endif::[]
+
+"{backend}" != "pdf"