First of all, you have to make sure that you have activated the option "signature of PDF files" in the company's settings:

Then, you have to set up the digital signature in the user's certificates: 

In the list of companies, select the users and then click on edit. 

Under the "Certificate" tab, click on the "Certificate" button.

By inserting your ID key into the computer, its information should be displayed in the list with the help of the "Refresh" button.

You can now export PDF files with their digital signature. 

Be careful though, as any subsequent modification of the PDF file will cause the signature to be lost!

Also note the incompatibility with the option of protecting PDF files with a password.