@RonaldLens analyzing the pega logs will help you pin point the process causing this.

In the past this error can be caused when SOAP service throws Entity too large exception when PDF is being sent.

Please verify the log of the server that is sending the response (i.e the server that you are sending PDF file to) for more error information.

 Verify logs of external system logs for further information and verify LimitRequestBody parameter in the server if the value is set too low. (if the server is tomcat, LimitRequestBody is the correct parameter, if server is something else, please verify corresponding parameter).LimitRequestbody is the parameter in Apache Tomcat which restricts size

Another thing to check:

Issue may be related to the Service URL settings and the authentication required by the external system . The URL parameters that are used for the authentication of the external system should not be empty during runtime. This leads to a denial of request from the external system because of the missing value. Try restart and clear cache for all the nodes

If you need further help analysing your logs you may want to log a support ticket via the MSP.  Please provide the reference here to allow us to help you track it.