If you are using an Apple computer you can download an Automator service
script that I made. It will enable you to select as many, or as few PDFs
as you would like to enable comenting in and will automatically enable
each PDF to "enable commenting". You can view the tutorial and download
the Automator Script here:http://www.youtube.com/watch?v=azRq6vRSVbsHope
this helps.