Represents Highlights PDF content. While the other annotations are drawn on top of the content, the highlight annotation is behind the text it highlights. In other words, it has to be inserted at a specific place in the ContentElementsUILayer. The precise place where the highlight should be placed is described in the PDF file with the BDC, MCID and EMC operators. At the moment, we have implemented a read-only rendering for all annotation types which draws the annotions separately from the page content. In the case of Highlight, the annotation is drawn below the other content. With this implementation, depending on the PDF content, the highlight annotation might be visible or it might not be because in some cases there is geometry content below the highlighted text which hides the highlight annotation appearance.