allow previewing pdf files

package.json

@@ -96,7 +96,7 @@
         "c9.ide.newresource": "#981a408a7b",
         "c9.ide.openfiles": "#7fa4a97fed",
         "c9.ide.preview": "#8f87ff2f6a",
-        "c9.ide.preview.browser": "#cec211a76a",
+        "c9.ide.preview.browser": "#c5b9a129de",
         "c9.ide.preview.markdown": "#bc846e1562",
         "c9.ide.pubsub": "#a85fb27eca",
         "c9.ide.readonly": "#e67bb593bd",