This occasionally comes up and the most likely cause is that you (or your students) don't have a fast internet connection or you are 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.
We'd suggest you follow best practice and compress your larger PDF files.
If you're looking for a really fast fix, possibly take a screenshot of the page you're wanting to share and upload the image into the whiteboard's lesson materials folder.