This is an option that the author can/really should use, but rarely does. If he does, it will be a note showing ALL of the time, not merely when you point at it, it will be distracting, impossible to overlook, ruining the design, and it will not be very precisely positioned. On the screenshot, I have made a note to the words American Heritage Dictionary - these words were marked when I wrote the note. See however how it seems to be positioned in the next line, and see also how big and yellow
it is. Maybe this are some of the reasons that so few are using the option. Another reason is of course that many pdf files were not first created in the pdf format, but maybe as Word files, and then converted - losing links & features.
However, if you want to, you can easily
add these notes yourself, one by one.
click for 719x198:references in PDF