The standard methods of getting a document from in the SDK (e.g. Record.GetDocument()) fetch the entire document from the document store before giving you access to the document.  DownloadNotifier allows you to:

• not write a file to the file system but to a stream,
• fetch the document in chunks rather than waiting for the entire document, and
• start downloading part way through a file.

I just updated the SDK docs to include some information on using it, also, here is a video if you want to watch me use it.