Acrobat Reader superscipts in comments

I receive many reports in PDF format from students studying chemistry, on which I comment using Acrobat Reader. I frequently need to correct their typography, e.g. to show subscripts etc in my "sticky note" comments - such as Zn2+ [whose 2+ superscript has just been rejected and "corrected" by the community editing tools].

How do I input this directly into Acrobat? 

It is possible to create the subscript/subscript in Word then copy it across to the comment, but then I might as well ask the students to submit in Word to start with (but that causes other problems).

Thank you