1) To use the PDF Library you'd contact Datalogics (http://www.datalogics.com/) who handle licensing. It is a C/C++ library. Licensing and pricing is by negotiation, but think $$$$.
2) The Acrobat SDK is documentation, header files, etc. It does not add programming interfaces. But you need to install it to get the documentation and samples.
3) Use Acrobat. The interfaces you get for free are naturally limited, Adobe aren't interested in giving this away. The Acrobat SDK is fundamentally a marketing tool for Acrobat.