You can only call the Acrobat API from one thread.
Also, it sounds like you are running this application in at least a "server like" environment. Acrobat is no technically suitable for that, but even more important, it is not licensed for this use. Please check the EULA for more information about what you can and cannot do with Acrobat.
If you want to use a solution that can be multi-threaded, you will have to look at PDF libraries and frameworks that can do that. If you want to stick with Adobe's solution, take a look at the Adobe PDF Library (licensed via DataLogics: http://www.datalogics.com/products/pdf/pdflibrary/). There are of course 3rd party libraries available from other vendors.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com