Hi Anja,
Thanks for your question, this is mostly due to the fact that the PDF Document Generation service is running in the public cloud, therefore, to be able to use it, Bi-Directional communication is required, as mentioned in the docs here.
That being said, our PDF Document Generation service does support static IP address ranges, therefore, if your use case does allow for access to whitelisted public services, please submit a support ticket through Mendix support here with your deployment type, environment(s) and use case details, and we are happy to help you set it up if possible.
The documentation says this is only supported on the following platforms as it needs to talk back to the licence server. Mendix Cloud, Mendix Cloud Dedicated, Mendix for Private Cloud Connected, and On-Premises. Other deployment scenarios will be supported at a later stage.
https://docs.mendix.com/appstore/modules/document-generation/#limitations
I hope this helps.
Thank you for your answers. We are using the connected architecture.