title: Add interstitial to pdf library
body: <p>I try to create a book with pdf library i found <a href="https://github.com/barteksc/AndroidPdfViewer" rel="nofollow noreferrer">this library</a> , i created my book everything is ok , i just wanna add interstitial admob and show it every 30 seconds , i tried with handler runnable but it's still show just one time.</p>
  
  <pre><code>mInterstitialAd = new InterstitialAd(this);
  mInterstitialAd.setAdUnitId("ca-app-pub-3940256099942544/1033173712");
  mInterstitialAd.loadAd(new AdRequest.Builder().build());
  handler = new Handler();
  
  final Runnable r = new Runnable() {
      public void run() {
          if (mInterstitialAd.isLoaded()) {
              mInterstitialAd.show();
          }
          handler.postDelayed(this, 1000);
      }
  };
  handler.postDelayed(r, 1000);
  </code></pre>