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

namespace AddLineNumber
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a PdfDocument of instance
            PdfDocument doc = new PdfDocument();

            //Load a PDF document
            doc.LoadFromFile(@"G:\360MoveData\Users\Administrator\Desktop\sample.pdf");

            //Get the first page
            PdfPageBase page = doc.Pages[0];

            //Find the spcific text in the fisrt line
            PdfTextFind topLine = page.FindText("C# (pronounced See Sharp)", TextFindParameter.None).Finds[0];

            //Get the line height
            float lineHeight = topLine.Bounds.Height;

            //Get the Y coordinate of the selected text
            float y = topLine.Bounds.Y;

            //Find the spcific text in the second line
            PdfTextFind secondLine = page.FindText("language. C#", TextFindParameter.None).Finds[0];

            //Calculate the line spacing
            float lineSpacing = secondLine.Bounds.Top - topLine.Bounds.Bottom;

            //Find the specific text in the last line
            PdfTextFind bottomLine = page.FindText("allocation of objects", TextFindParameter.None).Finds[0];

            //Get the height of the chunks 
            float height = bottomLine.Bounds.Bottom;

            //Create a font
            PdfFont font = new PdfFont(PdfFontFamily.TimesRoman, 11f);

            int i = 1;
            while (y < height)
            {
                //Draw line number before a specific line of text
                page.Canvas.DrawString(i.ToString(), font, PdfBrushes.Black, new PointF(15, y));
                y += lineHeight + lineSpacing;
                i++;
            }

            //Save the document
            doc.SaveToFile("result.pdf");
        }
    }
}

Imports Spire.Pdf
Imports Spire.Pdf.General.Find
Imports Spire.Pdf.Graphics
Imports System.Drawing
 
Namespace AddLineNumber
    Class Program
        Shared  Sub Main(ByVal args() As String)
            'Create a PdfDocument of instance
            Dim doc As PdfDocument =  New PdfDocument() 
 
            'Load a PDF document
            doc.LoadFromFile("G:\360MoveData\Users\Administrator\Desktop\sample.pdf")
 
            'Get the first page
            Dim page As PdfPageBase =  doc.Pages(0) 
 
            'Find the spcific text in the fisrt line
            Dim topLine As PdfTextFind =  page.FindText("C# (pronounced See Sharp)",TextFindParameter.None).Finds(0) 
 
            'Get the line height
            Dim lineHeight As single =  topLine.Bounds.Height 
 
            'Get the Y coordinate of the selected text
            Dim y As single =  topLine.Bounds.Y 
 
            'Find the spcific text in the second line
            Dim secondLine As PdfTextFind =  page.FindText("language. C#",TextFindParameter.None).Finds(0) 
 
            'Calculate the line spacing
            Dim lineSpacing As single =  secondLine.Bounds.Top - topLine.Bounds.Bottom 
 
            'Find the specific text in the last line
            Dim bottomLine As PdfTextFind =  page.FindText("allocation of objects",TextFindParameter.None).Finds(0) 
 
            'Get the height of the chunks 
            Dim height As single =  bottomLine.Bounds.Bottom 
 
            'Create a font
            Dim font As PdfFont =  New PdfFont(PdfFontFamily.TimesRoman,11f) 
 
            Dim i As Integer =  1 
            While y < height
                'Draw line number before a specific line of text
                page.Canvas.DrawString(i.ToString(),font,PdfBrushes.Black,New PointF(15,y))
                y += lineHeight + lineSpacing
                i = i + 1
            End While
 
            'Save the document
            doc.SaveToFile("result.pdf")
        End Sub
    End Class
End Namespace