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)
{
#region Create PDF with security

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

// Add new document
Document document = engine.AddDocument();

// Append new page to the document
Page page = document.AddPage(PageSizeType.A3, PageOrientationType.LandScape);

// Create new drawing
Drawing drawing = page.AddDrawing();

// Add standard font
uint font = document.AddFontStandard(StandardFontType.Courier, FontEncodingType.WinAnsi);

// Set Active Font
drawing.SetActiveFont(font, 50, false, false);

// Draw Text
drawing.PlaceText(100, 100, 0, "Hello World!");

// Closing drawing on the page
drawing.Close();

document.AddProtection(
DocumentPermissionType.AssembleDocument |
DocumentPermissionType.CopyInformation |
DocumentPermissionType.ExtractTextAndImages |
DocumentPermissionType.FillAcroForm |
DocumentPermissionType.ModifyAnnotation |
DocumentPermissionType.ModifyContent |
DocumentPermissionType.Print | DocumentPermissionType.PrintHiResolution,

SecurityKeyLength._128Bit, "123", "123");

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

#endregion

#region processing existing PDF and remove "print high quality" permission

// Add new 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
document.RemoveProtection();
}

// re-add protection with the same permissions except "print high quality" permission
document.AddProtection(
document.Permissions ^ DocumentPermissionType.PrintHiResolution,
SecurityKeyLength._128Bit, "", "");

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

#endregion

}
}
}

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

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