Sign a Pdf document using AWS KMS

Question

I have created in AWS KMS Console a Asymmetric key, RSA_2048, for Sign and verify. I have created an IAM user to use this key and attached to it the following policy:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "kms:Sign",
            "Resource": "*"
        }
    ]
}

now in my java code im using the following method:

    public byte[] signPDFWithAWS(ByteArrayOutputStream pdf) {
        try {
            // user creadentials
            AWSCredentialsProvider credentialsProvider = new StaticCredentialsProvider(new BasicAWSCredentials(Config.accesskey, Config.secretkey));
            AWSKMS kmsClient = AWSKMSClientBuilder.standard().withCredentials(credentialsProvider).withRegion(Config.region).build();
            String keyId = "MY-KEY-ID";

            byte[] pdfBytes = pdf.toByteArray();
            SignRequest signRequest = new SignRequest()
                    .withKeyId(keyId)
                    .withMessage(ByteBuffer.wrap(pdfBytes));

            SignResult signResult = kmsClient.sign(signRequest);

            return signResult.getSignature().array();
        }
        catch (Exception ex) {
            logger.fatal("Fatalerror: ", ex);
        }
        return null;
    }

and when it gets to the kmsClient.sign(signRequest) part - it throws an exception:

com.amazonaws.SdkClientException: Unable to execute HTTP request: Broken pipe

what am I doing wrong?

How can I programmatically sign a pdf file so that it WONT show "Signature validity is unknown" in Adobe Reader app?

1. read documentation
2. read Sign a PdfDocument using the digital signature returned by AWS KMS answers but didnt understood.