Add signer logic

Select the signature field on the PDF.

Select the Field Logic section of the field editor panel.

Add the conditions required to fill in that signature field.

For example, if a PDF has two signature fields, one for a U.S. person and another for a foreign person, you would set signature field logic to only show the U.S. person signature field if the Webform indicates the person is in the U.S. and vice versa.