Thanks for taking interest in GroupDocs.Viewer for .NET.
In fact, GroupDocs.Viewer for .NET, starting from version 3.0.0, is a standalome backend document rendering API and it does not provide any built-in UI or document viewer controller. It means that you will have to create your own front end application to view/display the rendered documents. Hence, you can implement pre/post loading of the documents in your front end application which is actually not a feature of the backend API.
In case you have any confusion, please feel free to let us know.