Is my document uploaded?

No. The file is opened and converted inside your browser. It is never sent to a server.

Why not just open the CSV in Excel?

Excel guesses types on import, which strips leading zeros, turns long numbers into scientific notation and mangles non-English text. Converting first keeps the values exactly as written.

Could a CSV contain a harmful formula?

Text is written as literal text, so nothing in the CSV is treated as a formula in the resulting workbook.

CSV to Excel - Free Online Tool

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Convert CSV data into an Excel .xlsx file.