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You received a .xls file — the legacy Excel format from before 2007. It's a proprietary binary format limited to 65,536 rows and 256 columns. If you're working with an .xls file today, it's either an old archive, a government form that hasn't been updated, or an export from enterprise software that treats 2003 as current.
The .xls format used a binary structure called BIFF (Binary Interchange File Format) that was notoriously difficult for non-Microsoft software to parse correctly. Formulas, charts, and conditional formatting from the .xls era sometimes render differently in modern applications. The format also supported macros — and was a major vector for virus distribution before Microsoft added security warnings.
Microsoft Excel opens .xls perfectly and can convert to .xlsx (File → Save As). Google Sheets imports .xls files. LibreOffice Calc handles most .xls files well. If you have a collection of old .xls files, converting to .xlsx removes the row limit (1 million+ rows), reduces file size, and improves compatibility with modern tools.