Thanks, that worked out fine. Pity the documentation is not that good, but as a side effect I found out about generating a document from Xml or via document.Add.
Only difference is I used Server.MapPath to get the file, and saved the 'stamped' document to a MemoryStream that is then streamed to the user. Didn't use FormFlattening either - what is the advantage of using it?
Also, how do you change author, subject, keywords etc using PdfStamper/Reader?