When you open a document, a status icon appears
next to the signature, indicating if the signature is valid.
Click Signatures in
the navigation pane, and select the signature.
Right-click/Control-click and choose Show Signature Properties.
Use the various tabs and options in the Signature Properties
dialog box to resolve any signature issues. For example, if the
identity is unknown or unverified, click the Signer tab, and click
Show Certificate to determine if the certificate is trusted.