A preloaded document is loaded into the server’s primary memory to ensure quick access at all times. It will however, use up memory even when no user is accessing the document.
When you open any Qlikview application, you will note that the application charts take some time to calculate/aggregate. Preloading helps first user get into the application quickly since it is already sitting in Server. Based on user selections memory cache saves that chart state in server RAM so that next user will instantly access the chart while making same selection.
In any instance the document will have all the associated data which was in Qlikview server.