using System.Diagnostics;
using Bytescout.PDF;

namespace LaunchActionExample
{
    /// <summary>
    /// This example demonstrates how to create a button that launches an external resource.
    /// </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);

            // Create button
            PushButton button = new PushButton(20, 20, 120, 25, "button1");
            button.Caption = "Launch sample.txt";

            // Create action that opens an external file
            LaunchAction launchAction = new LaunchAction(System.IO.Path.GetFullPath("sample.txt"));
            button.OnActivated = launchAction;
            page.Annotations.Add(button);

            // 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.txt

1234567890
abcdefghijklmnopqrstuvwxyz