This occasionally happens because either one or more of the participants do not have a strong internet connection or you're trying to share a large file which may 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 whiteboard.
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.