Hi there,
I don't know if this is already known, but Firefox 3.x (at least tested with 3.6) sends the mimetype "application/force-download" when uploading a PDF file. This makes ubiquo to not recognize correctly as "Document" the type of the asset uploaded, hence making the form selector for documents not to work correctly. In Firefox 4+ the problem has been resolved.
If you google for "application/force-download firefox 3" you'll see that more people is having problems with CMS-alike software, proposing as a solution adding the "application/force-download" as a valid mime type for pdf documents.