In previous versions of Acrobat or Reader (before 9.0), you can point the HTTP Submit button to a file:// URL on a network/local file system. The local file was “downloaded” using the browser’s plug-in. This behavior is not the intention of the HTTP Submit button. So, it’s been locked down in Acrobat and Reader 9 and later.
The HTTP submit button is used to send the form’s data to a server once the form has been filled. The resulting PDF can then be displayed to the user.
You can find more information to this security restriction under “app” and then the “methods” tab, and then under the launchURL method: