Change form-data files from scripting in a test flow

Hi, im trying to test a flow of services where one of them creates a pdf in Base64 and the other one has to take it to make some validations, it takes the pdf in a form-data file body. My question is: Can i take the Base64, transform it to pdf, download it to my files directory where postman will take it and then run the validation service to validate the pdf downloaded just putting the name of the file in the body request? or how can i change dynamically the form-data considering that im taking the pdf in Base64 from another service?