While most can open my .pdf attachments, I have one client who cannot. However, they can if sent from another person in my office. I am using Adobe Reader X1. Can anyone help?

While most can open my .pdf attachments to emails, I have one client who cannot.  However, they can open and read if sent from another person in my office who using the same Program as I do.  I am  using Adobe Reader X1.  Can anyone help?