How to solve this problem?
AsyncPostBackTimeOut is a property of ScriptManager of Ajax. You can change time out length. To solve this problem I have increased the time out of AsyncPostBackTimeOut property.
For example, for exporting 100 pdf I need 20 minutes. So, I will use maximum time out limit 20 minutes which is exactly 1200 seconds. My code is like below.
<asp:ScriptManager ID="ScriptManager1" AsyncPostBackTimeOut="1200" runat="server" />