Retool Support just pointed me out to this related thread.
I just followed same idea and built the request body from scratch in a code node.
return `
--__________boundary__________
Content-Disposition: related; name="input"
Content-Type: application/json
{"notes": "Sample Notes 5"}
--__________boundary__________
Content-Disposition: related; name="evidence-file"; filename="Testing document.pdf"
Content-Type: application/pdf
Content-Transfer-Encoding: base64
${jsGenerateFile.data?.[0]?.data}
--__________boundary__________--`
Then on the actual dedicated Paypal resource node:
And I finally received a successful response.