Download VeryPDF PDF Editor at www.verypdf.com and open it, then add a PDF file to it, next is the appearance when VeryPDF PDF Editor is opened.
click Edit Comment, some icons are enabled
click 
, thus in the PDF file of VeryPDF PDF Editor, a note sign is added, then right-click the note sign
choose Properties, then a window pops out
do the operation as the illustration indicates, choose Circle option, then the shape of the note sign has been changed
the color can be changed according to your like