In general you should be able to check response headers. if PDF is not generated you should be able to see html page that is affected by error. When PDF is generated, you will get file so headers would be different.
It's not possible to validate PDF content.
So was this possible in Browser or HTTP Monitor? I'm trying to understand if we can at least validate in a Browser monitor if the PDF did open or not. I don't see a way to validate headers in a browser monitor.