What are some alternatives?
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
FastHelp - FastHelp is a Windows Help File Generator that produces online and offline documentation.
MadCap Flare - Documentation for Any Audience, Language or Format
Doxter - Software for creating, organizing and publishing project documentation
Manula - Manula is an online software for creating user manuals.
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.