This .NET class wraps the AcApDocManager ObjectARX class. 

The DocumentManager object is the container for all the document objects (there is one document object for each drawing that is open and being edited).

Autodesk.AutoCAD.ApplicationServices.DocumentCollection

Public sealed Class DocumentCollection
Inherits MarshalByRefObject
Inherits ICollection

public sealed class DocumentCollection : MarshalByRefObject, ICollection;