using System.Diagnostics;
using Bytescout.PDF;

namespace FileAttachmentAnnotationExample
{
    /// <summary>
    /// This example demonstrates how to attach a file to PDF document.
    /// </summary>
    class Program
    {
        static void Main()
        {
            // Create new document
            Document pdfDocument = new Document();
            pdfDocument.RegistrationName = "demo";
            pdfDocument.RegistrationKey = "demo";

            // If you wish to load an existing document uncomment the line below and comment the Add page section instead
            // pdfDocument.Load(@".\existing_document.pdf");

            // Add page
            Page page = new Page(PaperFormat.A4);
            pdfDocument.Pages.Add(page);

            // Attach file
            FileAttachmentAnnotation fileAttachment = new FileAttachmentAnnotation("sample_attachment.txt", 20, 20, 20, 20);
            fileAttachment.Icon = FileAttachmentAnnotationIcon.Paperclip;
            page.Annotations.Add(fileAttachment);
            
            // Instruct PDF viewer application to show attachments pane on startup
            pdfDocument.PageMode = PageMode.Attachment;
            
            // Save document to file
            pdfDocument.Save("result.pdf");

            // Cleanup 
            pdfDocument.Dispose();

            // Open result document in default associated application (for demo purpose)
            ProcessStartInfo processStartInfo = new ProcessStartInfo("result.pdf");
            processStartInfo.UseShellExecute = true;
            Process.Start(processStartInfo);
        }
    }
}

sample_attachment.txt

1234567890
abcdefghijklmnopqrstuvwxyz