PDF Splitter

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using iTextSharp.text;
using iTextSharp.text.pdf;
using System.IO;
using System.Diagnostics;

namespace SzyApp
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        string pdfFilePath = "";
        string outputPath = "";

        private void SplitAndSaveInterval(string pdfFilePath, string outputPath, int startPage, int interval, string pdfFileName)
        {
            using (PdfReader reader = new PdfReader(pdfFilePath))
            {
                Document document = new Document();
                PdfCopy copy = new PdfCopy(document, new FileStream(outputPath + "\\" + pdfFileName + ".pdf", FileMode.Create));
                document.Open();

                for (int pagenumber = startPage; pagenumber < (startPage + interval); pagenumber++)
                {
                    if (reader.NumberOfPages >= pagenumber)
                    {
                        copy.AddPage(copy.GetImportedPage(reader, pagenumber));
                    }
                    else
                    {
                        break;
                    }
                }

                document.Close();
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if(txtFolder.Text != "" && txtInterval.Text != "" && txtPDF.Text != "")
            {
                int interval = int.Parse(txtInterval.Text);
                int pageNameSuffix = 0;

                PdfReader reader = new PdfReader(pdfFilePath);

                FileInfo file = new FileInfo(pdfFilePath);
                string pdfFileName = file.Name.Substring(0, file.Name.LastIndexOf(".")) + "-";

                for (int pageNumber = 1; pageNumber <= reader.NumberOfPages; pageNumber += interval)
                {
                    pageNameSuffix++;
                    string newPdfFileName = string.Format(pdfFileName + "{0}", pageNameSuffix);
                    SplitAndSaveInterval(pdfFilePath, outputPath, pageNumber, interval, newPdfFileName);
                }
                MessageBox.Show(pdfFileName + " was successfully split into " + pageNameSuffix +
                    " files, which are located at " + outputPath, "Success!");
            } else
            {
                MessageBox.Show("You're missing one of the required fields.", "Error");
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            OpenFileDialog openFileDialog1 = new OpenFileDialog();
            openFileDialog1.Filter = "PDF Files|*.pdf";
            openFileDialog1.Title = "Select a PDF File";

            if (openFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                pdfFilePath = @openFileDialog1.FileName;
                txtPDF.Text = pdfFilePath;

                PdfReader pdfReader = new PdfReader(pdfFilePath);
                int numberOfPages = pdfReader.NumberOfPages;
                lblPages.Text = "This file is " + numberOfPages + " pages long.";
                txtInterval.Text = "" + numberOfPages / 2;
            }
            else
            {
                MessageBox.Show("Please select a PDF file.", "Error");
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            FolderBrowserDialog folderBrowser = new FolderBrowserDialog();
            folderBrowser.Description = "Select a Folder";

            if (folderBrowser.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                outputPath = @folderBrowser.SelectedPath;
                txtFolder.Text = outputPath;
            }
            else
            {
                MessageBox.Show("Please select an output folder.", "Error");
            }
        }

        private void lblSource_MouseClick(object sender, MouseEventArgs e)
        {
            Process.Start("https://benbcompsci.wordpress.com/2018/11/19/c-split-a-pdf-file/");
        }
    }
}