ComCleanupExtensions.cs

public static class ComCleanupExtensions
{
    public static ComProxy<TResource> WithComCleanup<TResource>(this TResource resource) 
        where TResource : class
    { 
        return new ComProxy<TResource>(resource);
    }
}

ComProxy.cs

public class ComProxy<TResource> : IDisposable where TResource : class
{
    private readonly TResource target;

    public ComProxy(TResource target)
    {
        this.target = target;
    }

    public TResource Resource
    {
        get { return target; }
    }

    public void Dispose()
    {
        if (target != null && Marshal.IsComObject(target))
        {
            Marshal.ReleaseComObject(target);
        }
    }
}

WordToPDF.cs

public void Convert(string sourcePath, string resultPath)
{
    object sourcePathRef = sourcePath;

    using (var application = new Application().WithComCleanup())
    using (var documents = application.Resource.Documents.WithComCleanup())
    using (var document = documents.Resource.Open(ref sourcePathRef).WithComCleanup())
    {
        document.Resource.ExportAsFixedFormat(
            resultPath,
            WdExportFormat.wdExportFormatPDF,
            From: 0,
            To: 0,
            IncludeDocProps: false,
            CreateBookmarks: WdExportCreateBookmarks.wdExportCreateWordBookmarks);

        application.Resource.Quit(SaveChanges:false);
    }
}