Hi,
The problem is that your form is not Reader Enabled, so users with Reader cannot save the form. This is required for a 'submit as PDF' button.
You could change the submit type to 'submit as XML', which will work in a form that has not been Reader Enabled. You would then need to import the XML back into the form when you receive responses. This is probably the least expensive route.
LC Reader Extensions is a server product and there are additional costs involved in enabling a form through LC Reader Extensions. You should contact your local Adobe rep or check out here: http://www.adobe.com/uk/products/livecycle/readerextensions/
You can reader enable a form in Acrobat Standard (v9 or above) or using Acrobat Professional (v8). However there are licensing restrictions. See EULA.
I have a summary of deployment options here, which shows what features are available in your form depending on what the user has, whether the form is enabled, and how it was enabled.
http://www.assuredynamics.com/index.php/category/portfolio/deploying-forms-in-acrobat-and-reader/
Hope that helps,
Niall
