Open the PDF in Adobe Acrobat or Adobe Acrobat Reader
Go to the Tools menu
Select Certificates
Click Digitally Sign
Draw a signature box where you want the signature to appear
Choose a digital ID from the list
If prompted, create or import a digital ID
Enter the digital ID password
Review the document details
Click Sign
Save the signed PDF
Verify the signature status in the signature panel
If needed, click Validate Signature to confirm it is trusted and valid