They'd need Excel on their machine. I believe many Macs do have something compatible these days. I'd suggest creating a very simple Excel document then mailing it to your colleague to see if they can open it. I'd then get them to make a simple amendment, just type something into one cell. They then save it on their machine and email it back to you. If you can open it and see their amendment, then all's OK.
Sending it as a .pdf is, as others have suggested, only sending a picture - you're not sending the calculations. I'm trying to work out a way of you getting it to me. I work with Excel all the time and would be happy to help you sort it out. Just don't want to put my email address on here in public. If you can think of a way, I'd be glad to help.