Hi Pennyy
You can do it, but I don't advise it. When your papers arrive with the DWP , they are scanned into the online system, and therefore the highlighting can just appear as smudged and make it more difficult to read. Better to underline something - because it will still show when scanned and I would always refer to the highlighted sections of the letter where appropriate within the form.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
