Documentation for using JavaScript code inside a PDF file

I'm pretty sure it's an Adobe standard, bearing in mind the whole PDF standard is theirs to begin with; despite being open now.

My guess would be no for all PDF viewers supporting it, as some definitely will not have a JS engine. I doubt you can rely on full support outside the most recent versions of Acrobat (Reader). So I guess it depends on how you imagine it being used, if mainly via a browser display, then the majority of the market is catered for by Acrobat (Reader) and Chrome's built-in viewer - dare say there is documentation on whether Chrome's PDF viewer supports JS fully.

Probably you are looking for JavaScript™ for Acrobat® API Reference.

This reference should be the most complete. But, as @Orbling said, not all PDF viewers might support all of the API.

EDIT:

It turns out there are newer versions of the reference in Acrobat SDK (thanks to @jss).

Acrobat Developer Center contains links to different versions of documentation. Current version of JavaScript reference from Acrobat DC SDK is available there too.

The comprehensive place for Acrobat JavaScript documentation is the Acrobat SDK, which can be downloaded from the Adobe website. In the Documentation section, you will find all the material needed to work with Acrobat JavaScript.

To complete the documentation you may in addition get the specification of the JavaScript Core. My book of choice for that is "JavaScript, the Definitive Guide" by David Flanagan, published by O'Reilly.

Look for books by Ted Padova. Over the years, he has written a series of books called The Acrobat PDF {5,6,7,8,9...} Bible. They contain chapter(s) on JavaScript in PDF files. They are not as comprehensive as the reference documentation listed here, but in the books there are some realistic use-cases discussed in context.

There was also a talk on hacking PDF files by a computer scientist, given at a conference in 2010. The link on the talk's announcement-page to the slides is dead, but Google is your friend-. The talk is not exclusively on JavaScript, though. YouTube video - JavaScript starts at 06:00.