I suggest taking a look at ElementBuilder sample (included as part of the SDK):
http://www.pdftron.com/pdfnet/samplecode.html#ElementBuilder
For example:
using (PDFDoc = new PDFDoc("my.pdf")) {
ElementBuilder eb = new ElementBuilder();
ElementWriter writer = new ElementWriter();
Page page = doc.GetPage(1);
writer.Begin(page);
Element element = eb.CreateRect(new Rect(100, 100, 300, 400));
element.SetPathFill(true);
writer.WriteElement(element);
writer.End(); // save changes to the current page
...
}
Btw. it you want to redact PDF, the proper tool to use is 'pdftron.PDF.Redacrtor' (http://www.pdftron.com/pdfnet/samplecode.html#PDFRedact). Adding stuff on top of page does not really remove sensitive info.