What are some alternatives?
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Chmox - Chmox is a free (of charge), open source application for OS X which lets you read Windows Help...
Helpinator - WYSIWYG, single source, multiformat, multilingual help authoring tool with screenshot capture and annotation. CHM, PDF, WebHelp, QtHelp, JavaHelp, OracleHelp, WordPress. 249USD.
MadCap Flare - Documentation for Any Audience, Language or Format
Adobe RoboHelp - Adobe RoboHelp empowers organizations to deliver engaging help and policy content across mobile devices, formats, and platforms.
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.