Follow the below process to validate the signature in Adobe
Usually, the digital certificates are issued by a Root CA (Certification Authority).
If the Root CA that issued the signing certificate is not included in Adobe Trusted Identities, the digital signature is considered "not trusted" (but NOT invalid) when the document is opened in Adobe Reader (see example below).
This behavior has nothing to do with the signing engine (e.g. PDF Signer, Adobe Reader) but with the Adobe certificate validation procedure.
The user can validate the signature if the Root CA is already installed on Microsoft Certificate Store (see the section Validate the Signature using Windows Integration).
As an alternative, the recipient must manually add the Root Certificate of the signing certificate on Adobe Trusted Identities because only a few Root CA's are considered trusted by default by the Adobe certificate validation engine (See this article: http://www.adobe.com/security/partners_cds.html).
The digital signature in not trusted
1. Validate the Signature using Windows Integration
You can use this method if your digital certificate is issued by a Root CA already installed on Microsoft Certificate Store. Microsoft and Adobe use different Certificate Stores and different certificate validation procedures.
To see if your Root CA is installed on Microsoft Certificate Store, go to Start – Run – certmgr.msc
You can also import your Root Certificate here.
After you check that your Roor Certificate is installed, in Adobe Reader go to Edit menu – Preferences option – Security tab – click on Advanced Preferences button – Windows Integration tab and check all checkboxes.
When the document is re-opened, the digital signature is considered valid.
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