This site seems to offer a lot of API's for conversion of files to pdf via REST.
To use the service in the modeler see: https://docs.mendix.com/howto7/integration/consume-a-rest-service
When you want to use such an external service you have to make a connection with it. Most of the time webservices are used for that to make the connection. On how to use REST see this webinar: http://ww2.mendix.com/expert-webinar-REST-Services.html