In Microsoft Excel for iPad or Microsoft Excel for Mac, when you try to open a workbook that has been saved in Strict Open XML format, you receive the following error message:
The workbook you are trying to open is an ISO Strict file
If you click OK in the message window, the file does not open.
The ability to save Microsoft Office files in Strict Open XML format was introduced in Microsoft Office 2013 programs.
This is expected behavior when you try to open Strict Open XML files in Excel for iPad or Excel for Mac.
Note This article also applies to Microsoft Excel for Mac for Office 365.