Using itextsharp (or any c# pdf library), how to open a PDF, replace some text, and save it again?

Using itextsharp (or any c# pdf library), i need to open a PDF, replace some placeholder text with actual values, and return it as a byte[].

Can someone suggest how to do this? I've had a look at the itext docs and can't figure out where to get started. So far i'm stuck on how to get the source pdf from a PDFReader to a Document object, i presume i'm probably approaching this the wrong way.

Thanks a lot