To add a highlight annotation to a PDF Document page.

doc = PDFDoc(filename)
page = doc.GetPage(1)

# Create a highlight
hl = HighlightAnnot.Create( doc.GetSDFDoc(), Rect(100,490,150,515) )
hl.SetColor( 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).