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As mnuttall87 asked, it is likely that the spreadsheet file would be the one like .xlsx, .xlsm or something like that. AFAIK, the standard MS office file formats such as .xls, .doc, .ppt were turned to license-free and went to public by Microsoft. But the extended MS Office formats generated from the latest MS Office whose file extensions have 'x' or something at the end as .xlsx, .docx, .pptx, etc. are not public yet. Third party apps like Polaris will have no problem for the standard files but it's not guaranteed for the extended formats from the latest versions of MS Office like Office 2013.
Try out Microsoft Office Mobile as emilioaponte wrote. Or in case the file is from your friends or direct acquaintances, ask them to save the file in the .xls format and to share the file. Some functions included in the file from the latest Office, if any, will not be maintained if saved in old file format but there's no differences in most cases.