Jims comment above works for Macs too.

You can take a screen shot of the specific music in the pdf by holding down these keys: command, ctrl, shift and 4. This turns the cursor into a cross that you can drag over the selected text. When you let go of the keys to selection is saved to the clipboard. Then open a new document in Word and choose paste from the edit menu. The selection is then inserted into the Word doc where you can crop and place as desired. Comments are then easily added below the pasted image. If you want to create a pdf again then go to the print menu and print the doc to the desktop as a pdf and you are on your way.

The same process works on the PC side too except I cant remember the key combination that copies direct to the clipboard (as opposed to just using the print screen key and then cropping etc in word/publisher)