Digital Signatures in Excel 2007 and Excel 2003

If you want to add a digital signature to your excel file, follow steps below.

Microsoft Office Excel 2007

Press the Microsoft Office Button > Prepare > Add a Digital Signature > OK.
Don't forget to save your Excel workbook in the Excel 2007 workbook format, before you add signatures.
If you get the warning message asking "Do you want to save the workbook as a Microsoft Office Excel Workbook?", click "Yes" and select the Excel Workbook (*.xlsx) format in the Save As dialog box.
When the workbook is saved, the dialog box "Get a Digital ID " is displayed. Choose the type of digital ID that you want to use, and then press "OK".
After you complete the before mentioned steps for the type of digital ID, the Excel workbook is signed.

Microsoft Office Excel 2003

Go to Tools menu > Options > Security tab > Digital Signatures > Add.
If you changed your Excel file and is didn't save it yet, or if you didn't save it in the Excel 2003 workbook format, the message "This workbook must be saved as a Microsoft Excel workbook before it can be digitally signed. Do you want to save the workbook?" appears.
You should save the file in the Microsoft Excel Workbook format to add the digital signature.
After your workbook is saved, the Select Certificate dialog box appears. Select the certificate to use, and click "OK".
Close the Digital Signatures dialog box (Click "OK").
You Excel workbook is signed.