In Acrobat XI Professional you add Reader extensions on this menu item:
File > Save As Other... > Reader Extended PDF.
There are three options there:
- Enable adding text in PDFs (that are not PDF forms)
- Enable commenting & measuring
- Enable More Tools (includes form-fill-in & Save)
If your users have Reader XI, they can fill in forms and add comments without Reader-extending the PDF.
Please see this page about features new in Reader XI: