How to add or remove a check mark in a box on a form

The main problem is the document does not have interactive form fields. So you can either add them yourself with Acrobat of use the typewriter tool (Tools > Content > Add or Edit Text Box) to add text and the stamp tool to add checkmarks. This is what the recently added EchoSign functionality provides, but the checkmark is new to Acrobat 11. You can instead use an uppercase X for the checkboxes, or use the the built-in checkmark or X stamp.