To digitally sign a PDF in Adobe Reader DC you need to install the software to recognise your Certificate.
In case you haven't installed the software see here instructions how to install your:
To sign in Adobe Reader DC please follow these steps:
- Open the document clicking on File and then Open
- Click on Tools
- Then click on Certificates
- Next in Digitally Sign
- You'll be prompted with a next window. Please click ok
- Then choose a square to put your digital signature
- Then choose your certificate and press Continue
- Next click Sign
- Save the document
- Then signature will be added to the document