Got an ADR yesterday, they were not able to open the Employment Reference Letter at their end so requested to re-upload in proper format. I uploaded the same PDF again as I was able to open it at my end, I cross checked by sending it to my friends email and she was also able to access the PDF.
Later on I sent an e-mail to CPC Ottawa e-mail ID mentioning about the ADR and attaching the PDF.
I also used CSE web-form to inform CIC that i have sent an email too after uploading the requested PDF on my CIC account.