maxRequestLength security problem

<httpRuntime executionTimeout="90" maxRequestLength="4096" useFullyQualifiedRedirectUrl="false" minFreeThreads="8" minLocalRequestFreeThreads="4" appRequestQueueLimit="100" enableVersionHeader="true" /> I have a function to upload pdf to server. I used setting above in web config to limit the file size to 4MB. Can I set a alert to inform user the file size over limit? Now the setting above will show "the web page is not available" and when I click back button and click other buttons, the button function is not working properly, all will show "the web page is not available". How to solve this? Thanks.