"Should I use an Index for my help file or technical document?"

You should always use an index for all long documents and help files, especially if they are in print format.

“How long?”

There are no hard and fast rules for that. It’s up to your personal judgment and/or what your client/manager asks for. It depends on the “document specs” or “documentation plan” as well as your personal judgment. But for any technical document that is over 10,000 words, I’d say it would not hurt to have an Index.

“How about Glossary links?”

Some technical writers use glossary links for the terms in their online help files. That takes care of the explanation function but not the navigation function of the Index.

If you’d like to know what a specific word means, you can click on the Glossary link and read it. But what if you don’t know what the significant concepts are to begin with? What if you don’t know where the significant Glossary links are located in your document? In those cases, even Search would not help because by definition one wouldn’t know what to search for.

In contrast, a well-designed Index will easily guide you to those concepts and items that really count.

Some users print the help file for easy desk reference. That’s another case when glossary links would be useless since they obviously do not work when printed. An Index, on the other hand, works equally well whether it’s online or printed.