This read-only property will be the image identifier for the current image, or an empty string if the viewer does not have a currently-open image. This property will always match the encodedDocumentIdentifier property, except that encodedDocumentIdentifier is base64-encoded so the control can safely use it in a URL or in JavaScript code.

Note that if you call the openPage function, this property will not change to match the new image identifier until the image is fully opened, which will occur just before the viewer raises the pageOpened event. Prior to that time, this property will continue to hold the value for the previously-opened image or an empty string if there is no previously-opened image. If you need the new value of this property before the viewer raises the pageOpened event, use the currentState property.

Reference