You are able to upload documents to a meeting, discussion, comment and a poll. You can also upload documents in the document center as an administrator or member. 

Please note, in the document center you can not upload a file from Google Drive/Dropbox/OneDrive. If you would like, you could download the document from either of these three document centers and then upload it from your computer with a link in the notes section.

To upload a document, select 'upload'

Then you will be prompted to select a file from your computer, title the document, add any additional notes and decide whether or not to also email the document: