This occasionally comes up and the most likely cause is that you either don't have a fast internet connection or your teacher is trying to share a large file so it can take a while to load.
Please be patient and if the file still doesn't load after a minute, please refresh the page. You may need to re-enter the classroom.
You can also suggest to your teacher that they reduce the PDF file size for next time by using this free PDF compress tool.
The teacher can possibly take a screenshot of the page they are wanting to share with you and upload the image into the whiteboard.