Firstly, the attachment would need to have the category of Roster in order to be returned. If you wanted attachments with that tag, you would want to include "post_tag=Roster" in addition to the attributes already in your shortcode. It sounds like you have assigned that category to the post that the PDFs are attached to, rather than the actual attachments. This won't work.
The category/tag functionality is poorly documented at this time and I apologize for that. I will be working on a much more descriptive explanation of this functionality in the coming month.
In response to using GET values, can you give me the usecase you are thinking of for this? I believe it may already be covered in pre-existing functionality.
Hope this helps, and let me know if you have any followup questions/comments!