To upload documents you have to:
1-Have presenter status in the BigBlueButton meeting.
By default the first Moderator that enters the meeting is given presenter status; to pass presenter status click to the left of the users name.
If you are a viewer you will need to request presenter status from the moderator.
2-Ensure your documents are under the maximum size restrictions.
Maximum size is 30 MB or 150 pages per document; you can upload multiple documents into the BigBlueButton as long as they are under the size restrictions.
3-Ensure your documents are an accepted format.
BigBlueButton will accept many major document formats such as .doc .docx .pptx and .pdf we recommend converting your documents to .pdf for best results.
Select the "cloud" icon in the lower left hand corner of the presentation window, then choose "select file".
You will then be prompted to select a file from your system, once you have selected your chosen file you will want to "upload" it to the BigBlueButton.
Depending on the size of your document it may take several moments for it to convert and finally upload to the meeting.
See the short GIF below: