Working with Numbers on a Mac computer is great if everyone you work with also uses Numbers. However, if you need to share your spreadsheet with a coworker or client who uses Microsoft Excel, then you will need to convert your file to the XLS format that Excel uses. This is easiest using Numbers itself because there is no native way for Microsoft Excel to import or to read the Numbers file format.
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Save vs. Export
As you are creating your spreadsheet in Numbers you may be tempted to format your file with borders, colors, merged cells or other techniques to make the data easier to read. It should be noted that much of this formatting will only be maintained if you save your file in native Numbers format. In order for Microsoft Excel to read your file, you must click "File" then "Export" and select "Excel..." from the menu. This will create a new version of your Numbers file in XLS format. Exporting your file may cause some loss of original formatting. Things such as column widths, merged cells and perhaps even some data formats like decimals and currency may be lost in the export. In general, it is best to keep your spreadsheet as simple as possible in order to avoid major problems in exporting.