If you want to prove that a document, workbook, or presentation you’re sending or sharing really does come from you and hasn’t been changed by anyone since you finished it, you can include an invisible digital signature by attaching a digital certificate that verifies who you are. You’ll need to have a digital certificate issued to you to be able to digitally sign a document, workbook, or presentation. In most cases, digital certificates are issued through a third-party certificate provider or a corporate network.
With your document, workbook, or presentation completed and saved, and with the insertion point where you want the digital signature to appear, click the Office button, point to Prepare on the menu, and choose Add A Digital Signature from the gallery to display the Sign dialog box.
If you want, type a note stating the reason for signing the file.
Verify that the digital signature is the one you want. If it isn’t, click Change, and, in the Select Certificate dialog box, select the certificate you want. Click OK.
Click Sign. Close the document, workbook, or presentation, and then distribute or share it.
To verify that the digital signature is valid, open the file, click the Office button, point to Prepare, and choose View Signatures from the gallery to display the Signatures pane.
In the Signatures pane, point to the signature name, click the down arrow that appears, and choose Signature Details to display the Signature Details dialog box to verify the signature. Close the dialog box when you’ve finished.
"Signing a Document or Workbook with a Visible Signature" for information about obtaining a digital certificate to digitally sign a document, workbook, or presentation.