You can use JDIC for the Desktop integration rather than writing your own .dlls
You can use simply the open method of the Desktop Object to open a Powererpoint Document. This Method uses the default Application associated with Powerpoint files.
See:
https://jdic.dev.java.net/nonav/documentation/javadoc/jdic/org/jdesktop/jdic/desktop/Desktop.html#open(java.io.File)
For License questions see https://jdic.dev.java.net/