ติดตั้ง
PM> Install-Package iTextSharp -Version 5.5.13.1
using iTextSharp.text.pdf; using System.IO; namespace ConsoleApp1 { class Program { static void Main(string[] args) { string ipFilename = "original.pdf"; string opFilename = "encrypted.pdf"; string password = @"21042020"; using (PdfReader reader = new PdfReader(ipFilename)) using (FileStream fs = new FileStream(opFilename, FileMode.Create, FileAccess.Write, FileShare.None)) using (PdfStamper stamper = new PdfStamper(reader, fs)) { stamper.SetEncryption(PdfWriter.STRENGTH40BITS, password, null, PdfWriter.ALLOW_COPY); } } } }