I think it's Microsoft's version of an Adobe PDF
I found:
PDF is a popular file format created by Adobe and microsoft introduced XPS as an alternative to PDF for creating and sharing documents in electronic file format.However, Microsoft’s XPS format is not that popular yet and not many have moved from using PDF to XPS format for creating and sharing documents.
XPS file format is platform independent, openly published, and is made available royalty-free by Microsoft. They have even integrated XPS-based technologies into Microsoft Windows Vista and Windows 7 operating systems as well as Office 2007 and Office 2010 systems.
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