That's because iPhone / iPad does not natively support opening ms office documents. You may need to have an application that supports it. For example, check out this iPad app -
https://itunes.apple.com/us/app/documents-free-mobile-office/id306273816?mt=8[
^]
Once this is installed, you should be able open and view the xls files [Disclaimer - I have not used/tried this app]
Even better, check out various opinions -
here[
^]!