doc.addPage( page );
PDPageContentStream contentStream = new PDPageContentStream(doc, page);
//first fill the entire background with cyan
contentStream.setNonStrokingColor( Color.CYAN );
contentStream.fillRect( 0,0, page.findMediaBox().getWidth(), page.findMediaBox().getHeight() );
//then draw a red box in the lower left hand corner
contentStream.setNonStrokingColor( Color.RED );
contentStream.fillRect( 10, 10, 100, 100 );