You don't necessarily need Microsoft Excel to work with an .xlsx file Reviewed by Ryan Perian This article explains what XLSX files are, how to open them, and which programs are able to convert ...

If your Excel files are opening in Notepad on Windows PC, here's how you can reset the settings to make .xls, .xlsx, etc files open in Excel.

If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.