Did I miss something or is the answer to this question harder to track down than it should be?
As documented in the ‘BizTalk Server 2006 Installation Guide – Multiserver.doc’, Document Explorer 2005 is a dependency for the BizTalk 2006 documentation. So the operations team on my current engagement asks me where it comes from… and I respond that that is a good question and that I’ll have an answer for them in a couple of minutes.
After a few tens of minutes I’ve inferred that it is part of the Visual Studio 2005 installation, based on this document: http://msdn.microsoft.com/vstudio/support/AdminReadme/default.aspx ‘Help on Help’ leads me to believe that it could be (in the future?) installed from multiple products.