Hmm, I'm not sure about the pdf thing, but it has a fairly small number of core features[0], which are all documented and can be browsed off-line, and searched through from within the minimalistic editor that it comes with.

You could try looking at the source code of the documentation[1]. It uses PHP to render the site, although the actual documentation of the API is XML[2]

[0] https://processing.org/reference/

[1] https://github.com/processing/processing-docs

[2] https://github.com/processing/processing-docs/tree/master/co...