The end-user must verify and trust the Symantec Root Certificate before the signature will display a green checkmark or trusted.
To resolve this issue, perform the following steps:
1. Double-click signature field
2. Select Signature Properties > Show Certificate
3. From the left pane, select "VeriSign.." > Trust tab > Add to Trusted Identities
4. Once added, the signature should show valid. (green checkmark)
Note: Due to security issues, certificates are not automatically trusted. Symantec recommends to always review certificate information first before trusting any certificate. Once the certificate is trusted, all signed document will be trusted until a new certificate is issued (renewal or replacement).