Hi,
Have you looked at iText or iTextSharp to manipulate PDFs?
I saw this blog some time ago:
here...[
^] it is about working with images in PDF files using c# and iTextSharp.
iText if you like Java, or iTextSharp if you prefer .Net, will also let you add watermarks, annotations etc... google it and you'll see there's a million examples available.
Otherwise converting a PDF file to an image is very complex.