Version 1.0 of the XPS (XML Paper Specification) Essentials Pack is now available for download. It includes an XPS viewer, an XPS document writer, and iFilter/iPreview extensions to enable XPS search and thumbnails.
XPS is a fixed-layout document format that is supported by Office 2007 through an optional free download that enables XPS and PDF support. XPS documents use the Open Packaging Convention, so you can use the System.IO.Packaging API to read and write them. The parts themselves use XPS schemas instead of Open XML schemas, but the document architecture is based on OPC's standards for the structure of parts, relationships, and content types.