WordConverter.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Microsoft.Office.Interop.Word;

namespace Converter
{
    class WordConverter : IConverter
    {
        private Application application;

        public WordConverter()
        {
            this.application = new Application();
            this.application.ScreenUpdating = false;
            this.application.DisplayAlerts = WdAlertLevel.wdAlertsNone;
        }

        ~WordConverter()
        {
            // casting to _Application to prevent type conflicts
            ((_Application)this.application).Quit(WdSaveOptions.wdDoNotSaveChanges);
        }

        public void ToPDF(string sourceFilename, string destinationFilename)
        {
            Document document = this.application.Documents.Open(
                FileName: sourceFilename,
                ConfirmConversions: false,
                ReadOnly: true,
                AddToRecentFiles: false,
                Revert: true
            );

            document.ExportAsFixedFormat(
                OutputFileName: destinationFilename,
                ExportFormat: WdExportFormat.wdExportFormatPDF,
                OpenAfterExport: false,
                OptimizeFor: WdExportOptimizeFor.wdExportOptimizeForPrint,
                BitmapMissingFonts: true
            );

            ((_Document)document).Close(WdSaveOptions.wdDoNotSaveChanges);
        }
    }
}