Question

I have used UIDocument, but I don’t understand the advantage over using Archiving? Assuming you don’t want to use iCloud.

Answer 1

From Apple’s docs: http://developer.apple.com/library/ios/#documentation/uikit/reference/UIDocument_Class/UIDocument/UIDocument.html

Applications that make use of UIDocument and its underlying
architecture get many benefits for their documents:

• Asynchronous reading and writing of data on a background queue. Your
  application’s responsiveness to users is thus unaffected while reading
  and writing operations are taking place.
• Coordinated reading and
  writing of document files that is automatically integrated with cloud
  services. Support for discovering conflicts between different versions
  of a document (if that occurs).
• Safe-saving of document data by
  writing data first to a temporary file and then replacing the current
  document file with it.
• Automatic saving of document data at opportune
  moments; this mechanism includes support for dealing with suspend
  behaviors.

You get nice integration with NSUndoManager, too