Main

I would suggest to extract the two case branches' code into separate methods. After that your top-level functionality could be simplified like this:

using var originalPdfStream = new MemoryStream(buffer);
using var modifiedPdfStream = new MemoryStream();

var pdfReader = new PdfReader(originalPdfStream);
var pdfDocument = new PdfDocument(pdfReader, new PdfWriter(modifiedPdfStream));

EditFirstPage(pdfDocument);
EditSecondPage(pdfDocument);

pdfDocument.Close();
byte[] modifiedPdfInBytes = modifiedPdfStream.ToArray();

• I've renamed your variables to better express the intent (like stream >> originalPdfStream)
• I've used C# 8's using declaration to reduce code indentation
  • If you are using older C# version then you can only use here using statement

First page

void EditFirstPage(PdfDocument pdfDocument)
{
    var canvasPage = new PdfCanvas(pdfDocument.GetFirstPage());
    canvasPage.BeginText()
            .SetFontAndSize(PdfFontFactory.CreateFont(Common.Font), Page1.FontSize)
            .MoveText(Page1.Version.X, Page1.Version.Y)
            .SetFillColor(Common.FillColor)
            .ShowText(Common.VersionNumber)
            .EndText();
}

• I've moved all the hard-coded values into a dedicated class called Page1
  • Those constants that are shared between page 1 and 2 have been moved to a class called Common

static class Common
{
    public const string Font = StandardFonts.HELVETICA_BOLD;
    public static readonly Color FillColor = ColorConstants.WHITE;
    public const string VersionNumber = "4.2021";
}

static class Page1
{
    public const int FontSize = 34;
    public static class Version
    {
        public const int X = 735, Y = 248;
    }
}

Second Page

void EditSecondPage(PdfDocument pdfDocument)
{
    var canvasPage = new PdfCanvas(pdfDocument.GetPage(2));
    canvasPage.BeginText()
        .SetFontAndSize(PdfFontFactory.CreateFont(Common.Font), Page2.FontSize)
        .MoveText(Page2.Version.X, Page2.Version.Y)
        .SetFillColor(Common.FillColor)
        .ShowText(Common.VersionNumber)
        .EndText();

    var rectangle = new Rectangle(Page2.Disclaimer.X, Page2.Disclaimer.Y, Page2.Disclaimer.Width, Page2.Disclaimer.Height);
    var canvas = new Canvas(canvasPage, rectangle);

    var paragraph = new Paragraph()
        .Add(Page2.Paragraph.Disclaimer)
        .SetFont(PdfFontFactory.CreateFont(Page2.Paragraph.Font))
        .SetFontColor(Page2.Paragraph.FontColor)
        .SetFontSize(Page2.Paragraph.FontSize)
        .SetTextAlignment(TextAlignment.LEFT);

    canvas.Add(paragraph);
    canvas.Close();
    
    ImageData imageData = ImageDataFactory.Create(Page2.MapImage.FileName);
    var imageRectangle = new Rectangle(Page2.MapImage.X, Page2.MapImage.Y, Page2.MapImage.Width, Page2.MapImage.Height);

    canvasPage.AddImageFittedIntoRectangle(imageData, imageRectangle, true);
}

• I've renamed some variables and moved the hard-coded values into a dedicated class

static class Page2
{
    public const int FontSize = 21;
    public static class Version
    {
        public const int X = 760, Y = 573;
    }
    public static class Disclaimer
    {
        public const float X = 75, Y = 26;
        public const float Width = 883, Height = 81;
    }
    public static class MapImage
    {
        public const string FileName = "PathtoImage.png";
        public const float X = 111.318f, Y = 130.791f;
        public const float Width = 755.454f, Height = 432.094f;
    }
    public static class Paragraph
    {
        public const string Disclaimer = "Text";
        public const string Font = StandardFonts.HELVETICA;
        public static readonly Color FontColor = ColorConstants.BLACK;
        public const int FontSize = 12;
    }
}

The code which adds the version number to a specific page is redundant. You can define a helper method to that like this:

void AddVersionNumber(PdfCanvas canvasPage, int fontSize, double x, double y)
    => canvasPage.BeginText()
        .SetFontAndSize(PdfFontFactory.CreateFont(Common.Font), fontSize)
        .MoveText(x, y)
        .SetFillColor(Common.FillColor)
        .ShowText(Common.VersionNumber)
        .EndText();