The SDK to use depends on your environment and licensing considerations.
To send you to the right place... Is this for...
* Use on a single server?
* Use on multiple servers (sold product)?
* Use by individuals who own personal copies of Acrobat Pro?
* Self contained consumer software?
It is to be delivered integrated with our software that it is installed in multiple servers and with multiple web/windows computers doing digital signatures.
The windows client is developed in C++ unmanaged and the web client developed in C#.
In first stage, could be enough to integrate with our windows version (developed in C++ unmanaged). This is a thick windows client installed in each client computer (Acrobat Pro is not installed in each client computer).
Ok, that narrows the choices.Not the Acrobat SDK. (Details: Acrobat is not for server use. The Acrobat SDK is tools for automating Acrobat only).
So the Adobe PDF Library is the thing to look at. Licensing terms are by negotiation, there is no fixed price structure or at least not a published one. The Adobe library is pure C/C++.
All licensing is handled by DataLogics (http://www.datalogics.com/) who also provide value-added additional features such as support for managed languages; I don't know if these come at extra cost. So, contact DataLogics.