Hi Rama, I just want to start off with some clear terminology:
With AEM Forms 6.2 you have a couple of forms development choices: a) Create a fillable XDP form using Designer that can be rendered as both HTML and PDF; b) Create an Adaptive Form using the Adaptive Forms editor within AEM.
Now, if you're creating a new form and it's primary destination is the web and not offline or print then use Adaptive Forms. If you have existing PDF forms that you want to expose to the web quickly, then render your XDP's as HTML. That said, the experience with Adaptive Forms is much - much better from a user point of view and from a developer perspective, much more flexible.
From Adaptive Forms you do have the ability to generate a DOR but you'd likely want to create an XDP to be used as your final document of record (https://helpx.adobe.com/aem-forms/6-1/generate-document-of-record-for-non-xfa-based-adaptive-forms.h...).
From the web services perspective, yes, both PDF and HTML can call web services nicely. The architecture is a little different though. PDF web service calls are called directly from the client (Reader) but web service calls from HTML rendered from PDF is actually proxied to the server. Web services from Adaptive Forms are a snap because you have 100% control over the HTML that is rendered and the Client Libraries feature of AEM let's you create the JS that will make the AJAX calls for you (use JSON as all your return data to make your life easier).
