I've had a number of people ask me this question, my answer is based on implementing Confluence at two different companies and working with both Confluence and SharePoint for several years now. I'm also a Confluence administrator, so I am biased towards it as a solution.
As a pure documentation/collaboration platform, Confluence is a much better solution than SharePoint: It's relatively easy to set up and understand. The user interface is easy to grasp for most users. It is also very extensible (plugins) and reliable (particularly the 3.x releases). However, I caution you against using it as a file share (sharing PDFs, etc.), Confluence is not built to be a file server. If you want to share documents with Confluence, write them in wiki format from the beginning and everyone will be much happier in the long run.
SharePoint, on the other hand, tries to be many things to many people. I think this is it's biggest strength and it's biggest weakness. It's a file share, it's a web site, version control system, it integrates with Microsoft Office, it has (a pretty weak) wiki function. SharePoint's feature list is huge, yet all it's features don't appear to add up to anything terribly useful out of the box. You can make it do anything, but you have to be willing to invest the time and money to figure out what you want and implement it. Do not expect to install SharePoint and have it be useful immediately.
The other major flaw in SharePoint is its user interface. The out-of-box interface is just maddeningly confusing in my opinion. Again, expect to invest some time up front to make the interface work the way you need it to.
So, in summary, I think Confluence is a faster, easier solution for document collaboration and co-authoring. SharePoint is a very capable platform for file sharing, version control and tight integration with Microsoft Office, however you must have a clear implementation plan for SharePoint in order for it to be a useful, successful tool for your organization.