You would need to put the document viewer widget inside a dataview of the document that you want to view. One way would to use an action button that has a parameter of the same type as the data grid and you would pass that object to a page with a dataview. Then you can use the document viewer widget to display your document.
Another option would to use a dataview on the same page as the data grid and set the data source to listen to the datagrid. This way you can view a document by selecting a row in the datagrid.
Hope this helps!