Digtally Sign a PDF in Adobe Acrobat Reader using a Yubikey Token

This guide will show you how to sign a PDF document in Adobe Acrobat Reader with a document signing certificate from SSL.com.

These instructions were tested with a document signing certificate installed in slot 9c of a YubiKey 5 NFC FIPS, in Acrobat Reader Version 24.005.20320

Requirements

1. For customers who purchased a Yubikey from SSL.com, they can proceed to sign since the certificate is preloaded in their token.
2. For customers who purchased a Yubikey from Yubico, they will have to do an attestation before being able to install a SSL.com document signing certificate in their token. Refer to this guide for more information: Key Generation and Attestation with Yubikey

Steps to Sign

1. Make sure the YubiKey is inserted in your computer.
2. Open the document that you want to sign in Acrobat Reader.
3. Click the All tools tab followed by the View more link. 
4. Scroll down the options and select Use a certificate.
   
   
5. Select Digitally sign.
   
6. Adobe Acrobat will display some short instructions on how to configure the digital signature. Click the OK button to proceed.
   
   
7. Click and drag your mouse to select the signature area, then proceed to the next step of the signing process.
   
   
8. Adobe Acrobat will display all the available digital certificates. Click the radio button of the certificate that you want to use. It is possible that you have to choose from multiple certificates with the same name. If you are not sure which certificate you should be actually using, click the View Details link to  look for unique identifying information of the certificate, including its serial number.
   
   
9. In the Certificate Viewer box that appears, click your name, followed by the Details tab. Scroll down and check the serial number.
   
   

   Proceed to verify if the serial number of the certificate displayed by Adobe Acrobat is the same as what is indicated in your SSL.com order details page. 
   
   

10. After finalizing your certificate choice, click the Continue button to proceed with the signing.
    
11.  Click the Sign button.
    
    
12. Set the file name and click the Save button.
    
13. Enter your Yubikey PIN and click the OK button.
    
14. Success! You have applied a digital signature to your PDF.
    
15. If you click the signature box, Adobe Acrobat will confirm that the signature is valid and was actually created by you. Click the Signature Properties… button to view further information about the digital signature.
    
    
16. Any person who opens a PDF with a valid digital signature will be able to view the Signature Properties box which fully displays the identity of the signer, the time when it was signed, the validity of the document signing certificate, and that the document has not been modified since being signed.