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.
**The default value of the AsyncPostBackTimeOut property is 90 seconds.