private readonly string ApiKey = "YourApiKey";

        private async void ConvertionDocx2Pdf(string docFilePath, string filePathPdf)
        {
            var fileToConvert = docFilePath;
            using (var client = new HttpClient())
            {
                using (var content =  new MultipartFormDataContent())
                {
                    StreamContent streamContent;
                    using (FileStream fs = File.OpenRead(docFilePath))
                    {
                        streamContent = new StreamContent(fs);
                        var fileContent = new ByteArrayContent(streamContent.ReadAsByteArrayAsync().Result);
                        fileContent.Headers.ContentType = MediaTypeHeaderValue.Parse("multipart/form-data");
                        content.Add(fileContent, "File", Path.GetFileName(docFilePath));
                    }

                    using (
                       var message =
                           await client.PostAsync(string.Format("http://do.convertapi.com/word2pdf?ApiKey={0}",ApiKey), content))
                    {
                        var input = await message.Content.ReadAsByteArrayAsync();
                        var path = Path.Combine(filePathPdf);
                        File.WriteAllBytes(path, input);
                        Debug.WriteLine("The conversion was successful! The word file {0} converted to PDF and saved at {1}", fileToConvert, path);
                    }
                }
            }
        }