It contains the result of the response that the server sends back to the client. With Reader it will show an error or nothing because Reader is unable to convert the content of the response to PDF, unlike Acrobat.
To get different behavior, the server has to respond with something else that Acrobat/Reader can understand. This can be a HTTP 204 No Content type response, in which case it will be silent, or it can return an FDF that causes a simple popup dialog to appear, providing some useful feedback. This will require some modification to the server-side program that processes the form submission, but it's not too difficult.
I modified my server-side code to return HTTP 204. Now, upon form submission Acrobat (version 184.108.40.2069) still opens a completely-empty PDF. Running the same code in Reader (version 10.1.5.33) causes it to open a blank HTML file.
I would rather not use FDF since I am coding in PHP, FDF is no longer included in the PHP distribution and I don't have control over the PHP environment my site is running in.
Is there something weird about my settings that I am missing? Would it be easier to just build a SOAP web service to get my desired behavior?
The FDF functions that PHP had were only a wrapper around Adobe's FDF Toolkit, which is available but no longer supported. For what it's worth, the FDF that you'd return is very simple so it's easy to construct.
For the problem with the 204 response, it can work. Here's a previous thread that may help: http://forums.adobe.com/thread/1076091