Web Platform for creating User Manual

I have developed a web / browser based application for a specific purpose.

I need to create a user manual, and I could easily do it using word or something similar, converting it to PDF and deploy it on the site for download. The manual would contain text, images and possibly links.

I would rather have something like a CMS that is targetted at creating content to be served as a user manual. Something that can be searched, organized in a specific way, indexed and whatever else might be interesting for users to have.

I could use something like Drupal, Wordpress or other well known CMS, with or without plugins for it, or a more specialized CMS for that usage if it exists.

I would prefer something open source and php based so I can install it in an existing server.

What would you suggest?