1. Do you have the Acrobat SDK?
2. Is this for
* a single user, running interactively, who owns Acrobat?
* a single user, running interactively, who has the free Adobe Reader?
* a background task for a user who owns Acrobat?
* a web server (ASP, PHP etc.)?
* some other server process?
* something else?
3. Do you want the highlighting to be transient (just on screen) or to become a permanent part of the PDF?