I highlight academic documents using the text tool which looks like a capital I with elongated horizontals. When the text is really an image, I find that tool changes to a circle, which can be pulled across text or down the side. Either tool is fine for me. Now I have a document where this works on one (right) page but not on the left (facing) page, where I can't highlight except on a figure (which is an image of a graph), not on the body text below. The document is not read-only and I have just edited 3 other documents like it as usual with no trouble. I understand abot OCR, text and image. What is going on?