Is it permissible to mark the Mushaf with a pencil, pen, or felt-tip pens to highlight error locations while reading, to aid in memorizing the Quran?

The first is to safeguard the Mushaf from underlining or anything else that disfigures its appearance. Underlining with an ink pen or a highlighter disfigures it. If there is a need to underline, it should be done with a pencil, because it is easy to erase with a clean eraser that leaves no trace.