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.

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.

Likewise, the venerable .xls Excel spreadsheet format gave way to .xlsx, and PowerPoint .ppt files became .pptx files.

How to import Microsoft Excel spreadsheets into Apple Numbers This procedure works with .xlsx and .xls file formats. You can also import .csv and tab-delimited files.