What are some alternatives?
When comparing Adobe RoboHelp and HelpNDoc, you can also consider the following products
MadCap Flare - Documentation for Any Audience, Language or Format
WinCHM - Creating CHM file, Make CHM file, Web Help. WinCHM Pro - Help authoring software (CHM, WebHelp, DOC, PDF) . With this powerful tool you can not only create chm file from scratch, but also convert a set of existing HTML files to a chm file.
Dr.Explain - Dr.Explain software is a help file authoring tool.
Helpinator - WYSIWYG, single source, multiformat, multilingual help authoring tool with screenshot capture and annotation. CHM, PDF, WebHelp, QtHelp, JavaHelp, OracleHelp, WordPress. 249USD.
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
CHM Editor - Best solution to edit CHM files