AFAIK, The Document View SDK doesn't have the ability to edit PDF files.
DC View SDK doesn't currently support the ability for you to draw shapes or rectangles. What is currently available is using the pencil tool. There isn't a current way to extend the viewer to add rectangles or other features and save it back to the PDF.
At the moment we support three annotation tools namely, 'Note', 'Highlight' and 'Free Hand Drawing'. We do plan to support more annotation tools including shapes perhaps this year itself. Please watch this space for latest development and upcoming features of Adobe DC View SDK.