public class PdfFormatter : MediaTypeFormatter
{
    #region Constants and Fields

    private const int ChunkSizeMax = 1024 * 10;

    #endregion

    #region Constructors and Destructors

    public PdfFormatter()
    {
        SupportedMediaTypes.Add(MediaTypeHeaderValue.Parse("application/pdf"));
    }

    #endregion

    #region Public Methods

    public override bool CanReadType(Type type)
    {
        return false; // can't read any types
    }

    public override bool CanWriteType(Type type)
    {
        return type == typeof(byte[]);
    }

    public override Task WriteToStreamAsync(Type type, object value, Stream writeStream, HttpContent content, TransportContext transportContext)
    {
        Task t = new Task(() => WritePdfBytes(value, writeStream));
        t.Start();
        return t;
    }

    #endregion

    #region Methods

    private static void WritePdfBytes(object value, Stream writeStream)
    {
        byte[] buffer = value as byte[];
        int offset = 0;

        while (offset < buffer.Length)
        {
            int chunkSize = Math.Min(buffer.Length - offset, ChunkSizeMax);
            writeStream.Write(buffer, offset, chunkSize);
            offset += chunkSize;
        }
    }

    #endregion
}