Active the corresponding link in navigation menu by scrolling the content.

<script type="text/javascript">
//<![CDATA[

  $(function() {


      $(window).scroll(function() {
          var threshold = $(window).height() / 4;

          var active_section;
          var active_section_top = -Infinity;

          $(".scrolling-document-section").each(function(index, section) {
              var section_top = $(section).offset().top - $(window).scrollTop();
              if (section_top > active_section_top && section_top < threshold) {
                  active_section_top = section_top;
                  active_section = section;
              }
          });

          if (active_section) {
              $("#menu-explore-features a").removeClass("active");
              $("#menu-explore-features a[href$='#" + $(active_section).attr("id") + "']").addClass("active");
          }
      });

      $(window).scroll();
  });

//]]>
</script>