Convert .docx file into .pdf file in sp2013 using wordautomation services

<a href="https://msdn.microsoft.com/en-us/library/office/ff181518(v=office.14).aspx"></a>

using System;
using System.Security.Permissions;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Utilities;
using Microsoft.SharePoint.Workflow;
using Microsoft.Office.Word.Server.Conversions;

namespace WordTopdf.ConversionLibrary.PDF_EventReceiver1
{
    /// <summary>
    /// List Item Events
    /// </summary>
    public class PDF_EventReceiver1 : SPItemEventReceiver
    {
        /// <summary>
        /// An item was added.
        /// </summary>
        public override void ItemAdded(SPItemEventProperties properties)
        {
            base.ItemAdded(properties);
              // Verify the document added is a Word document
      // before starting the conversion.
      if (properties.ListItem.Name.Contains(".docx"))
      {
        //Variables used by the sample code.
        ConversionJobSettings jobSettings;
        ConversionJob pdfConversion;
        string wordFile;
        string pdfFile;
        string wordAutomation = "WAS";

        // Initialize the conversion settings.
        jobSettings = new ConversionJobSettings();
        jobSettings.OutputFormat = SaveFormat.PDF;

        // Create the conversion job using the settings.
        pdfConversion =
          new ConversionJob(wordAutomation, jobSettings);//[i am getting an error in here]

        // Set the credentials to use when running the conversion job.
        pdfConversion.UserToken = properties.Web.CurrentUser.UserToken;

        // Set the file names to use for the source Word document
        // and the destination PDF document.
        wordFile = properties.WebUrl + "/" + properties.ListItem.Url;
        if (properties.ListItem.Name.Contains(".docx"))
        {
          pdfFile = wordFile.Replace(".docx", ".pdf");
        }
        else
        {
          pdfFile = wordFile.Replace(".docx", ".pdf");
        }

        // Add the file conversion to the conversion job.
        pdfConversion.AddFile(wordFile, pdfFile);

        // Add the conversion job to the Word Automation Services 
        // conversion job queue. The conversion does not occur
        // immediately but is processed during the next run of
        // the document conversion job.
        pdfConversion.Start();

      }
    }
  }
}