When opening an .xlsx file in Excel for iPad, Excel for Mac or Excel for iPhone, you receive an error.

ERROR: "The workbook you are trying to open is an ISO Strict file"

 - You find this does not happen with all .xlsx files

 

Cause:

The file that will not open was created or saved using the newer Strict Open XML Spreadsheet (.xlsx) file type. The ability to save Microsoft Office files in Strict Open XML format was introduced in Microsoft Office 2013 programs.

ISO Strict file is not supported on Excel for iPad, Excel for Mac or Excel for iPhone

  

See screenshot below, there are two options for . xlsx in Excel 2013 to save: 

 Excel Workbook (*.xlsx)  and  Strict Open XML Spreadsheet (*.xlsx)

References:

New file format options in the new Office
http://blogs.office.com/2012/08/13/new-file-format-options-in-the-new-office/

Why can’t I open my file?
http://office.microsoft.com/en-us/excel-help/why-cant-i-open-my-file-HA103748560.aspx

2960660 Strict Open XML files do not open or save in Excel for iPad or Excel for Mac
http://support.microsoft.com/kb/2960660/EN-US