There isn't much formal documentation at the moment. I'm working to fix that, but it is taking time. I recently made a series of blog posts walking through the creation of a custom DataSource and the process is mostly the same as the Visualizers. My suggestion is to take a look at those posts (the first one is here: http://blog.andrewnurse.net/2008/02/29/DevelopingAnRSSDataSourcePartI.aspx and you can get the the rest of them from there). You should also check out the existing Visualizers which can be found in DesktopModules/Reports/Visualizers. If you download the Source package, you get the complete source code to the module and all the extensions (DataSources and Visualizers).

It should be possible to use the framework to develop a Visualizer that output to Excel or PDF. You will need some form of library to write the actual Excel/PDF files, but the data is provided.

Andrew Nurse
DotNetNuke Core Team Member and Reports Module Project Lead
Microsoft Certified Professional Developer