how to use iTextSharp with existing wpf document?

I have a existing 开发者_开发技巧wpf document, how do I get an pdf via iTextSharp from this?

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A simple way, which is easy, but probably not the most efficient way is to render the WPF document to an image and then embed the image in a PDF using iTextSharp.

I have done it this way before successfully. Initially I tried to convert the control primitives (shapes) to PDF equivalents, but this proved too hard.