API2PDF runs on Google Cloud Run, a serverless platform in Google’s us-central1 data center. There is no static IP address that you can whitelist.

But the most appropriate method is to add a custom header to the request that you specifically lookout for and allow. In the documentation at https://api2pdf.com/documentation and expand the POST methods, you will find the attribute for extraHTTPHeaders.

Add your own key-value pair and whitelist based on header value.