Drop a before and an after file. Every changed value, added or removed cell, changed formula, and inserted or deleted row is listed below. Everything runs in your browser; the files are never uploaded.
Tidyfield Compare reads both workbooks with the same engine that powers the Excel add-in. Sheets are matched by name. Rows are matched automatically, so an inserted row shows up as one inserted row instead of hundreds of "changed" cells below it. You can switch to position matching or match rows by a key column (an ID) under Options. Formulas are compared as text; values are compared with optional tolerance, case and whitespace rules.
The page is static. Your files are read by JavaScript in your browser and are not sent anywhere. Close the tab and nothing remains.
.xlsx and .xlsm only (save .xls or .csv as .xlsx first). Password-protected files cannot be opened. Very large files depend on your browser's memory; 200,000 cells per sheet is comfortable, millions are not.