To add a document to Generic Content Control on a page you should do the following.
1. Upload several document in one of your libraries.
2. Create a page and drop Generic Content control on this page.
3. Open the control and from the toolbar and select "Insert a ducument"
4. There are two options - to upload the file or select form already uploaded files.
5. Onece you click "select from already uploaded files" you should see your document libraries form where you can select whic document you want to upload.
All the best,
the Telerik team