How can one use a JavaScript library in an Acrobat JavaScript script?

Any .js file you place in the application or user JavaScripts folder will be loaded at start-up, and all the functions in it will be available to all further scripts running within the application. Just be aware that it has to be in code JS-syntax, as most JS libraries were written for web-based applications, and that won't work if try to use them in Acrobat.

You can define functions at application or document level and use the functions in your script.