Adding a PDF document stored on the web to ArcGIS Online and sharing the document with a group used for the Maps and Apps Gallery application, it prevents all the items referenced by the Maps and Apps Gallery application from loading unless user adds a thumbnail to the document within ArcGIS Online.
Refer to the following Knowledge Base article: <a href="http://support.esri.com/en/knowledgebase/techarticles/detail/41078" target="_blank">http://support.esri.com/en/knowledgebase/techarticles/detail/41078</a>