Secure Form on Return

I have Acrobat Professional 8, and I would like to send a form to a group of people and have them fill it out and submit the data back to me. As the data may contain sensitive information, I would like to have the data encrypted. There is a check box that is grayed out that says "Secure Forms on Return". The Adobe tip says that I need a Digital ID, and it gives me a button to "Get Digital ID from Adobe Partner". When I follow that link, it takes me to a complicated maze of PDF security providers, all focused around digitally signing a document to confirm to an end user that it was created by me (not encrypting PDF form data). I have set up a Self-Signed Digital ID, and the box is still grayed out. As the GeoTrust certificate that the tip points me to is $595, and I am not certain that it would accomplish what I need, I would appreciate insight from anyone who has encountered this. Thanks.