public Stream GetSignedDocuments(string envelopeId)
        {
            var uriTemplate = SignatureApiUriTemplates.BuildUriTemplate(SignatureApiUriTemplates.GetSignedEnvelopeDocuments);
            var parameters  = new NameValueCollection
            {
                { "userId", UserId },
                { "envelopeId", envelopeId }
            };

            var template = new UriTemplate(uriTemplate);

            Uri    url       = template.BindByName(new Uri(BaseAddress), parameters);
            string signedUrl = UrlSignature.Sign(url.AbsoluteUri, PrivateKey);

            var request = new HttpRequestMessage {
                Method = "GET", Uri = new Uri(signedUrl)
            };

            HttpContent content = request.Content;

            _client.DefaultHeaders.ContentLength = (content != null && content.HasLength() ? new long?(content.GetLength()) : null);

            OnSendingRequest(request);

            return(_client.Send(request).Content.ReadAsStream());
        }