NOTE: In order to view the document with full PDF functionality DO NOT view the PDF document in PDF/A mode. Adobe 9.0 and 10.0 (i.e. Adobe X) default document view settings are PDF/A mode which disables full PDF functionality. To enable full document functionality (use of bookmarks, hyperlinks, internal documents links) follow these instructions:
Go to the edit drop down menu
Select preferences
In the categories menu select Documents
In the documents category go to PDF/A view mode, select NEVER.
This will enable full document functionality, including commenting.