Is there a detailed document available for the SDK that explains each api call in detail with information on their parameters?
The (vp sdk userguide.pdf) is too high-level.
Or is the sampleapp the only source for this level of documentation?

The sampleApp is unluckily the only source of information to see how to use the SDK interfaces.

Why? Programmers obviously spent a lot of time making Viewpoint... They did not write up a spec? How can there not be a complete SDK specification released by Viewpoint for its own product?

Well this is correct that programmers spent a lot of time to implement the plugin, and maybe not enough to write a technical spec on paper
you have to be aware that 99.9% of people using viewpoint are making content. so all the time invested in documentation is for content creation.
However I tend to think that the sample app is very well documented and is a very good base to learn how to use the SDK.
If you have any specific questions don't hesitate to post them, we will gladly help you.

Hi ,
I've created an application 3D and I want to use the SDK for export MTX and MTS files, can I use? Where I can download?

Thanks