C#

 private void button1_Click(object sender, EventArgs e)
        {
// for getting the items ToLower case
            string[] fajli = Directory.GetFiles(Application.StartupPath,"*.*",SearchOption.AllDirectories);
            string[] VsiFajli = Array.ConvertAll<string,>(fajli, delegate(string s) { return s.ToLower(); });

            listBox1.Items.AddRange(VsiFajli);

            for (int i = 0; i < listBox1.Items.Count; i++)
            {
                // checks if the file is .pdf
                if (listBox1.Items[i].ToString() == listBox1.Items[i].ToString().Remove(listBox1.Items[i].ToString().Length - 4, 4) + ".pdf")
                {
                    if (File.Exists(listBox1.Items[i].ToString().Remove(listBox1.Items[i].ToString().Length - 4,4) + "_1.pdf"))
                    {
                        string temp = listBox1.Items[i].ToString();
                        
                        listBox1.Items.Remove(temp);
                        listBox1.Items.Remove(temp.Remove(temp.Length - 4,4) + "_1.pdf");


                        //listBox1.Items.RemoveAt(i);
                    }
                }
                else // if its not pdf ...
                {
                    if (File.Exists(listBox1.Items[i].ToString().Remove(listBox1.Items[i].ToString().Length - 4,4) + ".pdf"))
                    {
                        string temp = listBox1.Items[i].ToString();
                        listBox1.Items.Remove(temp);
                        listBox1.Items.Remove(temp.Remove(temp.Length - 4,4) + ".pdf");
                        listBox1.Items.Remove(temp.Remove(temp.Length - 4, 4) + "_1.pdf");


                        //listBox1.Items.RemoveAt(i);
                    }
                }
            }
            MessageBox.Show("Done!");
        }

        private void listBox1_DoubleClick(object sender, EventArgs e)
        {
            if(listBox1.SelectedItem!=null)
            System.Diagnostics.Process.Start(listBox1.SelectedItem.ToString());
        }

        private void button2_Click(object sender, EventArgs e)
        {
// for removing all .gif items
            if(listBox1.Items.Count != 0)
            for (int j = 0; j < listBox1.Items.Count; j++)
            {
                if(listBox1.Items[j].ToString() == (listBox1.Items[j].ToString().Remove(listBox1.Items[j].ToString().Length-4,4) + ".gif"))
                    listBox1.Items.RemoveAt(j);
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
// copying the selected item to clipboard.
            if (listBox1.SelectedItem != null)
                Clipboard.SetText(listBox1.SelectedItem.ToString()) ;
        }