Although it may not answer all your requests, DokuWiki may be worth taking a look at.
As with other wikis, it has a simple syntax, and has version control to track revisions, generates table of contents, and a full-text search feature which can come handy for a help system.
You may want to evaluate the feature list to see if it will meet your needs.
Also, there seems to also be a good collection of avaialble plugins. Although I haven't used DokuWiki or its plugins, there seems to be plugins available for PDF export as well.