how to add comments to a locked document

This probably is an odd question to some. In fact, I don't know if I should post it in the security forum instead. But here goes...

I receive a lot of pdf documents via mass distribution. Some of them are locked against everything but printing while some are not locked at all. As a new acrobat 8 pro user, I'd like to use the commenting/highlighting features on all these documents. The objective of course is not to alter those documents but just to make notes in an electronic format just like you would on paper and perhaps share these electronic comments with my colleagues.

Is there a way to "add a layer" or something like that so I can "attach" my comments onto the document without actually "editing" the document?

Thanks.