using System.IO;
using iTextSharp.text.pdf;

namespace BatchRename
{
    class Program
    {
        private static string getTitle(PdfReader reader)
        {
            string title;
            reader.Info.TryGetValue("Title", out title); // Reading PDF file's meta data
            return string.IsNullOrWhiteSpace(title) ? string.Empty : title.Trim();
        }

        private static string getSubject(PdfReader reader)
        {
            string subject;
            reader.Info.TryGetValue("Subject", out subject); // Reading PDF file's meta data
            return string.IsNullOrWhiteSpace(subject) ? string.Empty : subject.Trim();
        }

        static void Main(string[] args)
        {
            var dir = @"D:\Path";
            if (!dir.EndsWith(@"\"))
                dir = dir + @"\";

            foreach (var file in Directory.GetFiles(dir, "*.pdf"))
            {
                var reader = new PdfReader(file);
                var title = getTitle(reader);
                var subject = getSubject(reader);
                reader.Close();

                string newFile = string.Empty;
                if (!string.IsNullOrWhiteSpace(title))
                {
                    newFile = dir + title + ".pdf";
                }
                else if (!string.IsNullOrWhiteSpace(subject))
                {
                    newFile = dir + subject + ".pdf";
                }

                if (!string.IsNullOrWhiteSpace(newFile))
                    File.Move(file, newFile);
            }
        }
    }
}