I'm not sure what kind of trouble you're having, but I'm new to Canvas and Big Blue Button, but not new to video conferencing or learning management systems. I have not been able to get a presentation to upload, even one blank slide from PowerPoint, converted to PDF.
I am the product manager for BigBlueButton. While I work at Blindside Networks, the company that provides hosting for Canvas Conferences to Instructure, and can't speak on behalf of Instructure, I can respond to your question.
I just tried upload a .PPTX and was able to upload it and see it convert. Can you try it again and, if you are still having problems, can your share your .PPTX here and we'll take a closer look.