Please note that GroupDocs.Viewer is a back-end API that provides nearly all possible features that can be performed at the back-end of a document viewer application. It doesn’t contain any built-in viewer control or the UI which makes it more flexible for you to build the applications considering your security requirements.
Since the API only provides the back-end rendering mechanism, you can control and implement these security measures in your front end application. Furthermore, using GroupDocs.Viewer, you would display the rendered document pages (in the form of HTML pages or images), therefore, the end-user would not be able to access/download the original document. Also, you can prefer to use the image-based rendering to restrict the copy/paste of your content.