Is invoking a WSDL and using the SOAP object my only alternative here?
Yes ....you woudl use a Submit button to call your servlet (no different than a web page). If you want to return something it is important to know how your PDF is being housed. If it is in a browser then you can return HTML and the browser will interpret the page. If you are in a standalone app like Acrobat/Reader then you must return a PDF response (otherwise the application will not know what to do with the HTML that you woudl normally send).
If you want the servlet to "communicate" with the form then the best way is to create a Web Service.
The submit approach is used when the user has finished with the form and wants to move the information onto the next step in the process.
OK, that makes sense. Is it possible to submit the document's contents as well as part of the web service request? Another operation I'd possibly require would be the ability to pass the document's contents and one or more parameters to a web server, and then deal with the corresponding response.
Yes but it is not straight forward to get the doc ument into a Soap response ....there was a post that described this not too long ago. Have a search and see if you can find it.
For those that are interested I think this is the link that you were referring to: http://forums.adobe.com/message/2039661#2039661
I was under the impression that Net.SOAP.connect or SOAP.connect would work in a Readers Extenstions-enabled environment but haven't had any success in using these yet. Can anyone point me in the right direction please?