You have the wrong URL signature.
You are getting the document from the "0" internal view and simply opening a document.
You need to use the following url structure
http://host/NSF/xpagename.xsp??documentId=19381C380ED54DC9862577BF005B0CF5&action=openDocument
Example
http://localhost/XPBCDemo.nsf/PersonFormXPage.xsp?documentId=19381C380ED54DC9862577BF005B0CF5&action=openDocument