How to adjust the page height of a pdf page in ITextsharp code

You can install Free Spire.PDF via nuget and then use the following code to reset PDF page size.

using Spire.Pdf;
using Spire.Pdf.Graphics;
using System.Drawing;

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

            PdfDocument pdf = new PdfDocument();
            pdf.LoadFromFile("Sample.pdf");

            PdfDocument newPdf = new PdfDocument();
            foreach (PdfPageBase page in pdf.Pages)
            {
                PdfPageBase newPage = newPdf.Pages.Add(PdfPageSize.A1, new PdfMargins(0));
                PdfTextLayout loLayout = new PdfTextLayout();
                loLayout.Layout = PdfLayoutType.OnePage;
                page.CreateTemplate().Draw(newPage, new PointF(0, 0), loLayout);
            }
            newPdf.SaveToFile("Result.pdf");
        }
    }
}

For more information, visit this link:

https://www.e-iceblue.com/Tutorials/Spire.PDF/Spire.PDF-Program-Guide/Page-Setting/How-to-change-PDF-page-size-in-C.html