Attempted to access an unloaded AppDomain.

Hi, I am displaying a PDF file while clicking a button. When i clicked the button i got the following error. How can i get rid of this error? Attempted to access an unloaded AppDomain. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.AppDomainUnloadedException: Attempted to access an unloaded AppDomain. Source Error: An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. Mallik