CombinePDF via SDK returns an exception during merge process

We are using Node SDK to call CombinePDF but we are getting the below error during the merge process:

Exception encountered while executing operation Rie: Source PDF is certified and cannot be processed.
at processTicksAndRejections (node:internal/process/task_queues:96:5) {
requestTrackingId: 'bc530dd6-ff29-4f01-b284-6ec74e39751c',
statusCode: 400,
errorCode: 'PDF_CERTIFIED'
}

const fileRefFromStream1 = PDFServicesSdk.FileRef.createFromStream(s3Stream1, 'application/pdf');
const fileRefFromStream2 = PDFServicesSdk.FileRef.createFromStream(s3Stream2, 'application/pdf');

combineFilesOperation.addInput(fileRefFromStream1);
combineFilesOperation.addInput(fileRefFromStream2);

await combineFilesOperation
      .execute(executionContext)
      .then(async (fileRef) => {
        // do some  processing but not getting called because of the exception
      })
      .catch((err) => {
        if (
          err instanceof PDFServicesSdk.Error.ServiceApiError ||
          err instanceof PDFServicesSdk.Error.ServiceUsageError
        ) {
          console.log('Exception encountered while executing operation', err);
        } else {
          console.log('Exception encountered while executing operation', err);
        }
        Promise.reject(err);
      });

 Please advise.