By default, a viewer displays one data field. A viewer can also display several fields, depending on the field selection at runtime. For different views of the same zone on a document, you can also assign the same field to several viewers.
What you see in the viewer depends not only on the viewer properties, but also on its dimensions. The viewer always covers the area specified in the properties, but actually shows a larger snippet of the page that matches the viewer dimensions. To obtain the best results, ensure that the viewer dimensions closely resemble the geometry of the area to display.