Manage PDF with indexs

Some of the PDF files we are opening show a transparent blue rectangle over every entry of the document index.

We would like to hide those blue rectangles, but we don't know how to manage them.

At the begining we thought that they were Annotations, but all the pages in the PDF have zero annotations.

Any idea how to handle this?

plz try setAnnotTransparency(0); in Global.default_config()

That worked like a charm. Thank you.
But if the indexes are annotations, why the following code didn't work?

reader.PDFOpen(doc, false, this);
for(int i = 0; i < doc.GetPageCount(); i++) {
Page page = doc.GetPage(i);
int annots = page.GetAnnotCount();
for(int j = 0; j < annots; j++) {
Page.Annotation annot = page.GetAnnot(j);
annot.SetHide(true);
}
}

I debugged the code and it seems that pages with indexes don't have any annotation.
Any clue?
I ask that because maybe I would like to process these indexes to show them in another place.