From a user perspective, I’m still fond of WinHelp/CHM. It’s a rather consistent and predictable format, with a standard hierarchical TOC, index, full-text search, and hyperlinking, distributed as a single file you can open on any Windows PC. The AutoHotKey documentation is a good example.

EPUB is a fairly modern counterpart to CHM. The old WinHelp file format (.HLP) was RTF-based as pointed out in a sibling comment.

Oh yes, but that ship sailed really fast. The idea would start from RTF files, but by the time we hit the market, the distribution channel would be gone.