IT service provider seeks content management system

We are an IT service provider and are looking for a content management system. This is intended to replace MS Word for software documentation.

The CMS solution should meet the following requirements:

• Single source publishing
• Various outputs (at least PDF and HTML)
• XML editor with DITA standard
• Windows compatible

This currently involves one software workstation. In a year's time, there may be up to three software workstations.