Set page margin and size

VSTO
Spire.Doc
Download Sample Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.Linq;
using Word = Microsoft.Office.Interop.Word;
using Office = Microsoft.Office.Core;
using Microsoft.Office.Tools.Word;
using System.Drawing;
namespace VSTO
{
    public partial class ThisAddIn
    {
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
            //Open document
            Word.Application wordApp = Application;
            wordApp.Documents.Open("../../Sample.docx");

            //Set Page Margins
            Word.Range range = wordApp.Application.ActiveDocument.Range(1, 2);
            range.PageSetup.TopMargin = 12.0f;
            range.PageSetup.BottomMargin = 12.0f;
            range.PageSetup.LeftMargin = 12.0f;
            range.PageSetup.RightMargin = 12.0f;

            //Set Page Orientation
            range.PageSetup.Orientation = Word.WdOrientation.wdOrientLandscape;

            //Set Page Size
            range.PageSetup.PaperSize = Word.WdPaperSize.wdPaperA4;
        }
        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {
        }

        #region VSTO generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }
        
        #endregion
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Spire.Doc;
using Spire.Doc.Documents;

namespace Spire.Doc
{
    class Program
    {
        static void Main(string[] args)
        {
            //Load Document
            Document document = new Document();
            document.LoadFromFile("../../Sample.docx");

            //Set Page Margins
            Section section = document.Sections[0];
            section.PageSetup.Margins.Top = 12.0f;
            section.PageSetup.Margins.Bottom = 12.0f;
            section.PageSetup.Margins.Left = 12.0f;
            section.PageSetup.Margins.Right = 12.0f;

            //Set Page Orientation
            section.PageSetup.Orientation = PageOrientation.Landscape;

            //Set Page Size
            section.PageSetup.PageSize = PageSize.A4;

            //Save and Launch
            document.SaveToFile("WordMargin.docx", FileFormat.Docx2010);
            System.Diagnostics.Process.Start("WordMargin.docx");

        }
    }
}

PageSetup.zip