- Is this a issue with a particular PDF file or with all the PDF that he wants to sign?
- Would you mind sharing the steps the client is doing to sign the PDF?
- Also what is the version of the Adobe Acrobat DC he is using? To check the version go to Help > About Acrobat and make sure you have the latest version installed. Go to Help > Check for updates and reboot the computer.
Please try the following steps and see if that helps
Launch Adobe Acrobat DC
Select Edit -> Preferences...
Select "Signatures" from Categories
Click "More..." button on Creation & Appearance block
Uncheck "Use modern user interface for signing and Digital ID configuration"
Click the "OK" button and then again click "OK" button on Preference dialog
If it doesn't work try to click the "Certificates" button, then Certify Visible Signature, then draw a new box in the field you want signed and see if that works.