For HTML2PDF and XML2PDF solution check our PDFDoc Scout ActiveX library instead

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using Bytescout.PDF;

namespace HelloWorld
{
class Program
{
static void Main(string[] args)
{

// Create main PDF Doc Engine
PDFDocEngine engine = new PDFDocEngine("", "");

// Add existing document
Document document = engine.AddDocument("HelloWorldWithSecurity.pdf");

// check if document is protected
if (document.Protected)
{
// check password
if (document.ValidatePassword("123") == PasswordValidationType.Invalid)
{
Console.WriteLine("Invalid password");
}
else // removing protection if password was correct
document.RemoveProtection();
}

// Save document
document.Save("HelloWorldWithSecurity_ProtectionRemoved.pdf");

}
}
}

Download example source code: bytescoutpdf_remove_protection_from_existing_pdf.zip (10 KB)

For HTML2PDF and XML2PDF solution check our PDFDoc Scout ActiveX library instead