C# PDF Viewer Library
How to preview, view PDF files in C# .NET Core WinForms Windows application?

using System.ComponentModel;
using RasterEdge.XDoc.PDF;

namespace RasterEdgePDFWinFormDemo
{
    public partial class Form1 : Form
    {
        // Image size of the item in the ListView.
        //  Maximum to 256 * 256.
        private Size _listItemImageSize = new Size(256, 256);

        private BackgroundWorker _bgWorker;
        private int _totalFileCount = 0;

        public Form1()
        {
            InitializeComponent();

            //  Initial
            this.Text = "Folder:";
            this.toolStripStatusLabel1.Text = "";

            //  Initial ListView.
            this.listView1.BackColor = SystemColors.ControlDark;
            this.listView1.View = View.LargeIcon;
            this.listView1.LargeImageList = new ImageList();
            this.listView1.LargeImageList.ImageSize = this._listItemImageSize;

            //  Initial a BackgroundWorder object for load previews of PDF files.
            this._bgWorker = new BackgroundWorker();
            this._bgWorker.WorkerReportsProgress = true;
            this._bgWorker.WorkerSupportsCancellation = true;
            this._bgWorker.DoWork += new DoWorkEventHandler(bgWorker_DoWork);
            this._bgWorker.ProgressChanged += bgWorker_ProgressChanged;
            this._bgWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bgWorker_RunWorkerCompleted);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (this.folderBrowserDialog1.ShowDialog() == DialogResult.OK)
            {
                // Reset the ListView.
                this.listView1.Items.Clear();
                this.listView1.LargeImageList.Images.Clear();
                String targetFolder = this.folderBrowserDialog1.SelectedPath;
                try
                {
                    if (String.IsNullOrEmpty(targetFolder))
                        throw new Exception("Selected folder path is null or empty.");
                    //
                    String[] filePaths = Directory.GetFiles(targetFolder, "*.pdf", SearchOption.TopDirectoryOnly);
                    if (filePaths == null || filePaths.Length == 0)
                        throw new Exception("No PDF file in the target folder.");
                    this.Text = "Folder: " + targetFolder + "\\";
                    this._totalFileCount = filePaths.Length;
                    this._bgWorker.RunWorkerAsync(new List<String>(filePaths));
                }
                catch (Exception ex)
                {
                    this.Text = "Folder: [Invalid]";
                    MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }
            }
        }

        private void bgWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            if (e.Argument == null)
                return;
            int percentProgress = 0;
            this._bgWorker.ReportProgress(percentProgress);
            List<String> filePaths = (List<String>)e.Argument;
            for (int i = 0; i < filePaths.Count; i++)
            {
                String filePath = filePaths[i];
                String fileName = Path.GetFileName(filePath);
                Image bitmap;
                try
                {
                    // Load PDF file and get Bitmap of the 1st page.
                    PDFDocument doc = new PDFDocument(filePath);
                    Bitmap pageImg = doc.GetPage(0).GetBitmap();
                    // Resize the page image to fit the size of the image list item.
                    bitmap = resizeImage(pageImg, this._listItemImageSize);
                }
                catch (Exception)
                {
                    // Return a dummy image if load PDF file failed.
                    bitmap = new Bitmap(1, 1);
                }
                percentProgress += 100 / filePaths.Count;
                this._bgWorker.ReportProgress(percentProgress,
                new { Image = bitmap, FileName = fileName, Index = i });
            }
            this._bgWorker.ReportProgress(100);
        }

        // Resize the source Bitmap object to the specified size with using fit height mode.
        private Image resizeImage(Bitmap origImage, Size canvasSize)
        {
            // Get the scale ratio of the image.
            float ratio = (float)canvasSize.Height / (float)origImage.Height;
            // Get the image width after resizing.
            float w = (float)origImage.Width * ratio;
            // Center alignment
            // Get the X position of the page image in the result image.
            float xShift = ((float)canvasSize.Width - w) / 2F;
            Bitmap result = new Bitmap(canvasSize.Width, canvasSize.Height);
            using (Graphics g = Graphics.FromImage(result))
            {
                // Set background to transparent.
                g.Clear(Color.Transparent);
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                //
                g.DrawImage(origImage, xShift, 0, w, canvasSize.Height);
                // Draw a DarkGray border for the ViewItem.
                g.DrawRectangle(new Pen(Color.DarkGray), 0, 0, canvasSize.Width - 1, canvasSize.Height - 1);
            }
            return result;
        }

        private void bgWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            if (e.UserState != null)
            {
                dynamic data = e.UserState;
                // Add new item to ListView.
                this.listView1.Items.Add(new ListViewItem(data.FileName, data.Index));
                this.listView1.LargeImageList.Images.Add(data.Image);
            }
            // Update progress bar and status label.
            this.toolStripProgressBar1.ProgressBar.Value = e.ProgressPercentage;
            int loaded = (int)((float)e.ProgressPercentage * (float)this._totalFileCount / 100F + 0.5F);
            this.toolStripStatusLabel1.Text = loaded + " files loaded - Total " + this._totalFileCount + " files";
        }

        private void bgWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if (e.Cancelled)
            {
                MessageBox.Show("Task Cancelled.", "Info", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
            else if (e.Error != null)
            {
                MessageBox.Show(e.Error.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
    }
}