Returns the document number of the current image. This is a read-only property.
Currently this property is only valid in Windows & Mac.
The document number, assigned by the Source, that the acquired data originated on. Useful for grouping pages together. Usually a physical representation, this could just as well be a logical construct. Initial value is 1. Increment when a new document is placed into the document feeder (usually tell this has happened when the feeder empties). Reset when no longer acquiring from the feeder.