Hi Mike,
Numbers is not Excel, and cannot work on Excel files.
Excel is not Numbers, and cannot work on (or open) Numbers files.
When you 'open' an Excel file using Numbers, you are actually importing and translating that file into a Numbers file—the only type of file that Numbers can actually edit.
If you then want to save a file that an Excel user can open and use in Excel, you must Export the Numbers file you are working on to the file format in which you want it saved. Numbers again translates the file, this time into the MS Excel file format, and attempts to save the result. macOS (or OS X, depending on the version being used notices that there is already a file with that name in the location you want to save this one, and warns you that doing so would overwrite that file. IF you don't want to keep the version of the Excel file as it was sent to you, then tell Save that it can overwrite the original. If you do want to keep the original, you will need to rename the file you are attempting to save, or save it to a different location, or give it both a revised name and a different location, before clicking Save.
Not all features of Numbers are supported in MS Excel.
Not all features of Excel are supported in Numbers.
Features that are present in both applications are not al supported using the same method.
Fonts, even some with the same name on both platforms, are not necessarily the same on Mac and on Windows.
Anything depending on those features or fonts will not be unchanged (or in many cases, even present) it the translation to the 'foreign' format.
If you need to regularly exchange files with co-workers using MS software, and want those files to make the transitions with minimum unintended change, your best route (without moving everything to Windows) is to use Office for Mac, which uses the same file formats as Office for Windows, and works with the native Word, Excel and Power Point files.
An alternative is to use the free LibreOffice or Apache OpenOffice. (Both do request donations to support future development.) The links will take you to the download sites for these applications.
Regards,
Barry
