We noticed the behaviour in some courses where a PDF is not loading (properly).
In the Files section of the course we uploaded several PDFs a while ago.
When trying to open them, it remains a blank sheet. As if there's no content there.
For some it started working all of a sudden, others remain empty.
We have tried in different browsers.
At first I thought it had to do with some rendering because canvas has its own PDF/docs engine.
But these files have been uploaded for more than 1 week already...