Problem with PDF attachments in calendar items

I have access to my manager's calendar in Outlook. We are both able to access his calendar on our iPhones. When I attach a PDF to a calendar item, I can see it without any problem on my phone. Unfortunately, he is unable to open the attached PDF. It just shows an image that has three lines of information -- file name, PDF document, and Zero KB. Any ideas how we can fix that for him?