This issue with the PDF not loading occasionally happens because of either:
the file is too large (we recommend files of less than 10MB so they reload quickly for future classes);
the internet connection is slow;
the user does not wait for the upload to finish. Large files may take a while to load with a slower speed connection (30 seconds - 90 seconds is possible)
Please be patient, and if the file still doesn't load after a few minutes, please consider compressing the file first before re-uploading the file.
How to compress your PDF for free:
We'd suggest you follow best practices 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.