Selecting Input and Output Formats : Understanding Output Formats : Microsoft HTML Help : Restrictions and Requirements for Microsoft HTML Help
 
Restrictions and Requirements for Microsoft HTML Help
The Microsoft HTML Help output format is recommended to produce standard help for 32-bit applications that run on a computer running Windows 95 or later. For more information, see “Requirements”. Review the following considerations when deciding whether to deliver HTML Help:
*Your audience cannot read the .chm file over a network or over the Web. The .chm file must be installed on the local computer.
*Some security settings can interfere with .chm file use.
*A .chm file supports many Web technologies, such as scripting languages, which also allow them to contain and transport viruses and security risks. For this reason, many email systems remove .chm files when attached to an email message.