Thanks for providing the required information.
In fact, GroupDocs.Viewer for .NET is a stand-alone, backend and UI less document rendering API that enables you to render the document in a viewable/displayable form. The API provides Html based rendering and image-based rendering.
Html based rendering: In Html based rendering, the API generates the Html representation of each page of the source document. You can easily manipulate the Html content of each page and save it as an Html file.
Image-based rendering: In image-based rendering, the API converts each page of the document into an image (PNG, JPG, BMP). You can easily save the image to your desired location and display it in your application to view the content of the document.
As the API provides the back-end features of document rendering and it does not contain any built-in UI, therefore, you will have to create your own front end application to display the rendered Html pages or images. Furthermore, GroupDocs.Viewer also allows you to render the document as PDF document.
We have also developed an open source document viewer application (in ASP.NET MVC) using GroupDocs.Viewer for .NET. You can download this application from here. To learn more about this open source document viewer application, please visit this documentation article.
As I mentioned previously, GroupDocs.Viewer is a backend API (available as a DLL) that allows you to render the source document in the form that can be viewed/displayed in the front end application. The documents that you want to preview can be located on the same machine where the application is running or they can be stored at external storage i.e. Amazon S3. GroupDocs.Viewer provides the IInputDataHandler interface to configure the external storage. You will find this documentation article helpful for the integration of external storages.
In case you would have any other question, please feel free to let us know.