I have created a form and placed checkboxes on the form for the users to check.  I called it checkbox1 and checkbox2 for this example.  Form works great - I'm able to check which one I need.
When I go to do merge to an existing PDF, I got to place the checkbox on the form and it shows that it will place an "X" but it does not.  It places the entire text string.  
I attached a copy of the "Fill Area Settings" - it shows in the sample it will put an "X" but the fill mode is not editable and is set to field content.  How can I change this?