Introduction

public static void ReplaceText()
{
  Console.WriteLine( "=== REPLACE TEXT ===" );
  var outputFileName = "ReplaceText.pdf";
  var outputPath = TextsSample.TextsSampleOutputDirectory + outputFileName;

  // Loads a PdfDocument.
  using( var pdfInput = PdfDocument.Load( TextsSample.TextsSampleResourcesDirectory + @"Two Page Text Only - from libre office.pdf" ) )
  {
    // Gets the first Page. 
    var page = pdfInput.Pages.First();

    var test = page.Text;

    // Set the text to find in a specific area of the page.
    var textSearchOptions = new TextSearchOptions( "License" ) { Area = new Rectangle( new Point( 200, 50 ), new Point( 400, 108 ) ) };

    // Get all the texts matching the search options in page 1.
    var textMatches = page.FindText( textSearchOptions );

    // Replace the found texts with a red CourierBoldOblique font of size 16 new text.
    var newTextStyle = TextStyle.WithFontAndBrush( pdfInput.Fonts.GetStandardFont( StandardFontType.CourierBoldOblique ), 16d, Brushes.Red );
    page.ReplaceText( textMatches, "Details", newTextStyle );

    // Saves to the output document.
    pdfInput.SaveAs( outputPath );
    Console.WriteLine( $"Info exported to path: {outputFileName}" );
  }
}