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I'm not able to open .xls file in web app. Size is 5mb.
Could there be version/ size problem?
Opening .xls file for Office Web Apps is not supported in SharePoint Online. You may need to convert .xls to .xlsx and open it again.
For more information about Office Web Apps, please refer to the link and download the document named Microsoft Office Web Apps Service Description:
How are things going? Is the problem resolved?