You can serialize the dynamic content to XAML string, send xaml to server, convert XAML to PDF using PDFNet - as shown in Xaml2Pdf (http://www.pdftron.com/pdfnet/samplecode.html#Xaml2Pdf) and return the result as part of the http response.
Some more related projects
· http://weblogs.asp.net/mehrantoosi/archive/2008/03/03/silverlight-s-xamlwriter.aspx
· http://projectsilverlight.blogspot.com/2009/10/silverlight-3-xamlwriter-v04.html