HTML would be the next best choice, ONLY IF you would serve them from a public web server. If you tried to bundle it with your app, all the files (and images (and stylesheets (and …) ) ) would make CHM look like a gift from gods.
That said, when actually bundled in the installation package, (instead of being served over the network), I found the CHM files to work nicely.
OTOH, another pitfall about CHM files: Even if you try to open a CHM file on a local disk, you may bump into the security block if you initially downloaded it from somewhere, because the file could be marked as “came from external source” when it was obtained.