Use the following steps to resolve the “At least one signature is invalid.” message.
Complete the initial configuration
■ 1. Download the root certificate that is on your computer by default
1-1. Go to [ Finder ] → [ Applications ] → [ Utilities ] → [ Keychain Access ].
1-2. From the Keychain Access [System Roots], find [VeriSign Class 3 Public Primary Certification Authority - G5], right-click to export and save the file.
■ 2. Register the saved file in Acrobat Reader
2-1. Open [Acrobat Reader] and click [Preferences].
2-2. From the panel on the left side, select [Signatures] → [Identities & Trusted Certificates] then click [More].
2-3. From [Identities & Trusted Certificates] click [Import] → [Browse], specify the file saved in step 1. and select [Import].
2-4. Click the [Edit Trust] button of [VeriSign Class 3 Public Primary Certification Authority - G5] to open the edit window.
2-5. Select [Trust] in the dialog box. Select the [Use the certificate as a trusted root] checkbox and click [OK].
This completes the required settings.
Close the file once, and then try opening the file again to check.
If the problem persists
If the steps above do not solve the problem, try the following method.
Launch Acrobat or Adobe Reader.
From the [Edit] menu, click [Preferences].
The [Preferences] dialog box is displayed.
a) From the panel on the left side, click [Trust Manager].
b) Select the [Load trusted certificates from an Adobe AATL server] checkbox.
c) Click [Update Now].
The [Security settings have been successfully updated.] dialog box is displayed.
Click [OK] to close the [Security settings have been successfully updated.] dialog box.
Click [OK] to close the [Preferences] dialog box.