I'm writing a converter from HTML to FlowDocument - an XSD that defined the elements and attributes of a FlowDocument would be invaluable, if anyone knows of one.
The Flow Document schema uses the namespace of Windows Presentation Foundation. You can find the XSD in C:\Program Files (x86)\Microsoft Visual Studio 10.0\Xml\Schemas (location can be slightly different depending on version of Visual Studio). It is called wpfe.xsd.
However, I cannot find FlowDocument or other elements defined there. In fact, I cannot find it in any of the XSD's defined there, so that is not really helping.
Microsoft redefined the target namespace in this document from http://schemas.microsoft.com/client/2007 to http://schemas.microsoft.com/winfx/2006/xaml/presentation, which is very unfortunate, because that means that the schema does not match even the target namespace. Some discussion on this can be found in this thread and in this Microsoft page on Silverlight (similar story).
Long story short, unless you can find it embedded in one of the .NET libraries, you may be out of luck and the closest you can get is XAML, which is defined in xaml2006.xsd in the same location.