Use an external service for this like the ones used in these two modules from the app store:
https://appstore.home.mendix.com/link/app/112333/Mendix/DocuSign-Connector
https://appstore.home.mendix.com/link/app/108973/eXp-Realty/Hellosign-Module
Or use the signature widget, use the image from there to create a second pdf and merge the pdfs with functions from the community commons module from the app store:
https://appstore.home.mendix.com/link/app/107984/Mendix/Signature
https://appstore.home.mendix.com/link/app/170/Mendix/Community-Commons-Function-Library