As Stéphane says, you should use an Apple Help Book (basically HTML in a certain structure).
However on the flip side, developing for Apple help has become a frustrating experience in the recent years; it likes to cache pages (in an undisclosed location) so when you think you've fixed a typo or something it never shows up.
Apple also used to recommend the use of Apple Script to provide automation from pages (i.e. Click to open Preferences window), but again in recent years their *security* drive has also made this a frustrating experience.
To the point where, this year I got so fed up, that like you I rolled my own Help viewer, and apart from not having a search function (yet) it solves the two issues I was facing. I'll come back and add a search function later this year.