I'm sorry but this is what I know. There is no FTP access to WordPress.com blogs and we cannot install plugins. I don't know of any workarounds but perhaps other Volunteers or Staff will be able to provide them.
What follows is what I know: The following processes are outlined particularly for users who wish to upload a document and add its download link to a blog post or page. If you are seeking to simply upload a document to your blog’s Media Library (no download link), you can check out Media -> Add New in your dashboard. http://support.wordpress.com/media/media-add-new/
If you want to embed the document itself, you will have to use a third party application like Scribd or Google Docs. Otherwise, the documents will appear as a download link.