I think that @michaelderekjones is right here about the payload structure.
From the docs, I believe the file needs to be base64 encoded and added to the raw JSON payload as a string on the file property.
file: The base64 encoded string of bytes containing the PDF file to be used.
There are tools like PDF to Base64 Encode - Online Tool that could help you to convert that file type. As thatβs a 3rd party online tool, I would make sure that what youβre not sending anything sensitive over to it.
If the data isnβt confidential, you can send it to Postman Echo which will return the base64 encoded string.
You should be able to parse the response and store the base64 string in a collection or environment variable which can be used in the request to the Pleasesign API.