Document viewer displays the document, its attachments and connected/inserted documents.
Its basic function is to provide view of a document in human friendly way so users can operate and easily view the document.
You can also switch between Image (human friendly view of a document) and Data tabs – where data is stored:

To edit data the document must not be signed or initialed. Use Actions menu and click on “Document change” to alter the data.
Setting up document view
To adjust document view in document viewer, use View menu in Menu bar or right-click on a document:
Full screen (ctrl+f)
Maximises document view into fullscreen
Document details(ctrl+d)
View or hide Document details pane on right side of document view.
Show thumbnails
Shows or hides Thumbnails pane on the left side of document view.
Page height
Sets document viewer size to fit page height.
Page width (ctrl+0)
Sets document viewer size to fit page width.
Zoom in (ctrl + +)
Zoom in document viewer size.
Zoom out (ctrl + -)
Zoom out document viewer size.
Ruler (ctrl + r)
Shows/hides ruler tool. See more about ruler tool here.