Hi,
Please use the following code:
//Set both password
PdfFileSecurity fileSecurity1 = new PdfFileSecurity("orignal.pdf", "encrypted.pdf");
fileSecurity1.EncryptFile("userpassword", "ownerpassword", PdfPrivilege.Copy | PdfPrivilege.Print, true);
//Remove both password
PdfFileSecurity fileSecurity2 = new PdfFileSecurity("encrypted.pdf", "nopassword.pdf");
fileSecurity2.DecryptFile("ownerpassword");
//Remove open document password
PdfFileSecurity fileSecurity3 = new PdfFileSecurity("encrypted.pdf", "noopenpassword.pdf");
fileSecurity3.ChangePassword("ownerpassword", "", "ownerpassword");
//Remove permission password
PdfFileSecurity fileSecurity4 = new PdfFileSecurity("encrypted.pdf", "nopermissionpassword.pdf");
fileSecurity4.ChangePassword("ownerpassword", "userpassword", "");
Thanks.