You can use Word's Track Changes features, one of which is Comments. On the Review tab, check the option you'd like in Balloons. You may wish to check Show only comments and formatting in balloons, and then uncheck Formatting under Show Markup. Place your cursor where you wish to make a comment, or highlight the text about which you wish to comment. On the Review tab, click New comment (or type Alt+R,C) to create a comment balloon, then type; your cursor will already be in the balloon.