exported PDF's unreadable

I've had 5 clients this week tell me that they cannot open the .pdf's I have created in Corel Draw. 

I have tried the Save As function, the Export Function, and the Publish to PDF function, and there doesn't seem to be a difference for the clients. 

Other posts have suggested using a different program to open the files, but I have not control over what a client uses to open them.

I am running Corel Draw 2017 ver.,  on Windows 10 Pro ver. 1709