使用itextsharp，为什么我的标题被覆盖，但只在第一页上？

Question

我正在使用itextsharp创建一个函数，该函数将标题表添加到PDF中，然后根据标题表调整边距，因此它不会被后面的文本覆盖。

然而，由于某种原因我无法理解，在第一页上，一行文字覆盖了标题，然后从第二页开始，边距是完美的。

我记得读过SetMargins调用从下一页开始生效;但是在第一页上更改边距，因为在打开文档之前进行了SetMargins调用。

那么为什么一行文本会覆盖第一页上的标题，而不是其他页面上的标题呢？毕竟，SetMargins调用只发生一次。

谁能告诉我为什么会这样？

以下是我用来创建pdf的代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using iTextSharp.text;
using iTextSharp.text.pdf;
using System.IO;
using it = iTextSharp.text;
using System.Diagnostics;
using System.Drawing;
using System.Data;
using Helper;

namespace Andrew
{
    public class AndrewFunctions
    {
        public void createPDF()
        {
            //Create new document
            Pdf doc = new Pdf(PageSize.LETTER, 15, 15, 15, 5);

            //Set font and color
            it.Font myfont = FontFactory.GetFont("Garamond");
            myfont.SetColor(51, 51, 255);

            //Create instance= of pdfwriter named test to write to file
            PdfWriter writer = PdfWriter.GetInstance(doc, new FileStream(@"C:\Users\Gabriela\Desktop\test.pdf", FileMode.Create));

            try
           {
                //Create header table
                PdfPTable header = new PdfPTable(1);
                header.WidthPercentage = 80f;
                header.TotalWidth = doc.Right - doc.Left;
                header.AddCell(new PdfPCell(new Paragraph("THIS IS SOME HEADER TEXT")));
                header.AddCell(new PdfPCell(new Paragraph("THIS IS SOME HEADER TEXT")));
                header.AddCell(new PdfPCell(new Paragraph("THIS IS SOME HEADER TEXT")));
                header.AddCell(new PdfPCell(new Paragraph("THIS IS SOME HEADER TEXT")));


                //Add heading before opening doc to account for margin changes
                doc.AddHeader(header, writer, doc);

                 doc.Open();

                //Add filler content to pdf
                for (int i = 0; i < 1200; i++)
                {
                    doc.Add(new Phrase("This is filler text", myfont));
                }
        }
        catch (it.DocumentException dex)
        {
            Console.WriteLine(dex.Message);
        }
        catch (IOException ioex)
        {
            Console.WriteLine(ioex.Message);
        }
        finally
        {
            //Close doc and writing stream
            doc.Close();
            writer.Close();
        }
    }

使用标题更容易理解差异，一些屏幕截图显示我的意思：

the glibc one

Header overwritten, page 1

命名空间Helper：

using System;
using System.Collections.Generic;
using System.Collections;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using iTextSharp.text;
using iTextSharp.text.pdf;
using System.IO;
using it = iTextSharp.text;
using OfficeOpenXml;
using OfficeOpenXml.Drawing;
using System.Diagnostics;
using System.Drawing;
using System.Data;
using System.Web;

namespace Helper
{
    public class Pdf : it.Document
    {

        public Pdf() : base() { }
        public Pdf(it.Rectangle pageSize): base(pageSize) {}
        public Pdf(it.Rectangle pageSize, float marginLeft, float marginRight, float marginTop, float marginBottom) : base(pageSize, marginLeft, marginRight, marginTop, marginBottom) { }

        public void AddHeader(PdfPTable table, PdfWriter writer, Document document)
        {
            //Change margins to adapt to the Header
            document.SetMargins(document.LeftMargin, document.RightMargin, document.TopMargin + table.TotalHeight, document.BottomMargin);            
            Header header = new Header(table);
            writer.PageEvent = header;
        }

        public class Header : PdfPageEventHelper
        {
            //Table used for the header
            public PdfPTable headerTable{ get; set; }

            public Header(PdfPTable table)
            {
                this.headerTable = table;
            }

            // Place the header on each page 
            public override void OnEndPage(PdfWriter writer, Document document)
            {
                 try
                 {

                      Rectangle page = document.PageSize;
                      float tableheight = document.TopMargin;

                      headerTable.WriteSelectedRows(0, -1, document.Left, page.Height - tableheight + headerTable.TotalHeight, writer.DirectContentUnder);
                 }
                 catch (DocumentException ex)
                 {
                     throw ex;
                 }
            }
        }
   }
}