This issue with the PDF not loading occasionally happens because either the file is too large (we recommend files of less than 10MB so they reload quickly for future classes) or the internet connection is slow or 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.
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.