To add a highlight annotation to a PDF Document page.

PDFDoc doc = new PDFDoc(filename);
Page page = doc.GetPage(1);

// Create a highlight
Highlight hl = Highlight.Create( doc, new Rect(100,490,150,515) );
hl.SetColor(new ColorPt(0,1,0), 3 );
hl.RefreshAppearance();
page.AnnotPushBack(hl);

Add or edit PDF annotations sample
Full code sample which shows how to add or edit PDF annotations (e.g. hyperlink, intra-document link, stamp, rubber stamp, file attachment, sound, text, free-text, line, circle, square, polygon, polyline, highlight, squiggly, caret, and ink).