This is my code that generates a ZIP file on the fly and writes a pdf (fetched from AWS S3 bucket) inside the ZIP file. If I write the ZIP file to the disk via os.create there is no issues but if I try to send the file via HTTP response I get gibberish data in my Postman response.

func admin_invoice_download_handler_wo_disk(w http.ResponseWriter, r *http.Request) {
    fileName := "sample.pdf"

    // Initialize AWS S3 Config
    cfg, err := config.LoadDefaultConfig(context.TODO(),
        config.WithRegion("ap-south-1"),
    )
    if err != nil {
        log.Println("AWS S3 config setup not successful. ", err)
    }

    // Create S3 service client
    client := s3.NewFromConfig(cfg)

    result, err := client.GetObject(context.TODO(), &s3.GetObjectInput{
        Bucket: aws.String("bucket-name"),
        Key:    aws.String("bucket-dir/sample.pdf"),
    })
    if err != nil {
        log.Print(err)
    }

    defer result.Body.Close()

    // body in byte
    body, err := io.ReadAll(result.Body)
    if err != nil {
        log.Print(err)
    }

    // creating a zip
    buf := new(bytes.Buffer)
    writer := zip.NewWriter(buf)
    f, err := writer.Create(fileName)
    if err != nil {
        log.Fatal(err)
    }
    _, err = f.Write([]byte(body))
    if err != nil {
        log.Fatal(err)
    }
    resp1 := &common_tools.MessageResponse{
        Message: "Invoice downloaded successfully. ",
    }
    err = writer.Close()
    if err != nil {
        log.Fatal(err)
    }

    w.Header().Set("Content-Disposition", "attachment; filename=invoices.zip")
    w.Header().Set("Content-Type", "application/zip")
    
    io.Copy(w, buf)
    json.NewEncoder(w).Encode(resp1)
}

This is the postman response

PKsample.pdf�wT�[�/�l@rVh@@���9眡AB�d�AA2�s�I@�(�$�E2�L���y��w����Z�ǸV��Ԯ���v���U{ê)+��'���`gK;��(�����X�[88ۀ ��N(70k� �6(70/�,.B9Yc��f�/=�����;�7i���eQ��v����      ��  A�Qn��V(7��(���l��r���"yg'w��(�����`�o���+.�jXڡ�~�+����n)��p޿���+�[����SЁ���?t�`8�OI�����B��1�z
����F!���`A>�=A!(�7��~�oє�CtQ���%G���� �����
�
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@q���'����v� ��('w�qvrG9�������i�6����;��w�pGɢ���Q �*����!���삛�+���8#�d��N����ex@M(����B��8��ⳅ1�Jʼn�˸/���Ĥw��.�UݗA��ս�p���Ԁ���S��Xf��mUS����c��=H�Iq�U(5b]���ҳ�}��V宀_i�u�1!J�8'���c.SW�g*���r�Ŧb��C҇o��������_���U��0$��KQ���-*��ٶe<�}���v�(�6Z}����^����������2#����2f�ͶL?b��`*��
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I tried creating the file on disk via os which works fine but whenever I try to send the same http file via HTTP response I get gibberish data in my Postman Response. I’m expecting a zip file which has my required pdf.