安装Spire.PDF for .NET;然后使用下面的示例C＃代码加密任何PDF文件：

Install Spire.PDF for .NET; then use the below sample C# code to encrypt any PDF files:

  static void EncryptPDF()
   {
   //Create a pdf document.
   PdfDocument doc = new PdfDocument();
   // Create one page
   PdfPageBase page = doc.Pages.Add();
   //Draw the page
   DrawPage(page);

   //encrypt
   doc.Security.KeySize = PdfEncryptionKeySize.Key128Bit;
   doc.Security.OwnerPassword = "e-iceblue";
   doc.Security.UserPassword = "test";
   doc.Security.Permissions = PdfPermissionsFlags.Print | PdfPermissionsFlags.FillFields;

   doc.SaveToFile("Encryption.pdf");
   doc.Close();

   //Launching the Pdf file.
    System.Diagnostics.Process.Start("Encryption.pdf");
}

< b>从PHP网站下载的PDF书籍

这些书籍通过加密下载或密码

i希望开发c＃app成为唯一可以打开这些书的程序

和其他PDF工具不能这样做

怎么样？< ; / b>

<b>PDFs books downloaded from PHP site
these books downloaded with encryption or password
i want to develop c# app to be the only program can open these books
and other PDF tools cannot do this
how?</b>

我也在找解密根本就没有大多数都是收费的

我找到了

我也在找解密 根本就没有 大多数都是收费的
I AM FIND