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.

How to Find Word & Excel Documents on Your Hard Drive. Word and Excel are part of Microsoft's Office suite of productivity software. If your small business has been using Office for a while ...

If Developer tab in Microsoft Office is missing, you can add Developer tab in Word, Excel, etc. via Settings, Group Policy, Registry Editor.