How to redact all occurrences of a specific text in a PDF file?

Using Syncfusion.Windows.PdfViewer;
using System.Collections.Generic;
using System.Drawing;
using System.Windows;

/// <summary>
/// Gets all the bounds of the text present in the PDF file.
/// </summary>
/// <param name="text">text to be searched</param>
/// <returns>The collection of page index and the bounds collection of the searched text</returns>
private Dictionary<int, List<RectangleF>> GetTextBounds(string text)
{
   text = text.ToLower();
 
   Dictionary<int, List<RectangleF>> textBounds = new Dictionary<int, List<RectangleF>>();
 
   for (int i = 0; i < pdfViewer.PageCount; i++)
   {
       List<RectangleF> bounds = new List<RectangleF>();
 
       // Extract text and its bounds from the PDF file.
       List<TextData> textDataCollection = new List<TextData>();
       string extractedText = pdfViewer.ExtractText(i, out textDataCollection).ToLower();
 
       int start = 0;
       int indexOfText = 0;
       int end = extractedText.Length;
       int count = 0;
 
       // Iterate and get all the occurrences of the given text.
       while ((start <= end) && (indexOfText > -1))
       {
           count = end - start;
           // Get the next index of the text to be searched 
           indexOfText = extractedText.IndexOf(text, start, count);
           if (indexOfText == -1)
               break;                  
 
           // Holds the bounds of the first character in the text.
           RectangleF startCharacterBounds = textDataCollection[indexOfText].Bounds;
 
           // Holds the bounds of the last character in the text.
           RectangleF endCharacterBounds = textDataCollection[indexOfText + text.Length - 1].Bounds;
 
           // Get the bounds of the whole text.
           RectangleF rectangle = new RectangleF(startCharacterBounds.X, startCharacterBounds.Y,
                        endCharacterBounds.X - startCharacterBounds.X + endCharacterBounds.Width,
                        startCharacterBounds.Height > endCharacterBounds.Height ? startCharacterBounds.Height : endCharacterBounds.Height);
                    bounds.Add(rectangle);
 
                    start = indexOfText + text.Length;
                }
                // Add to the collection if any text is obtained.
                if (bounds.Count > 0)
                    textBounds.Add(i, bounds);
            }
            return textBounds;
        }
 

        /// <summary>
        /// Marks the rectangle regions to be redacted in the PDF pages
        /// </summary>
        /// <param name="bounds">It has the collection of information about the page index and the bounds of the areas to be redacted</param>
        private void MarkRegions(Dictionary<int, List<RectangleF>> bounds)
        {
            if (bounds.Count > 0)
            {
                // Iterate the collection and mark regions
                foreach (KeyValuePair<int, List<RectangleF>> textBounds in bounds)
                {
                    pdfViewer.PageRedactor.MarkRegions(textBounds.Key, textBounds.Value);
                }
                pdfViewer.PageRedactor.EnableRedactionMode = true;
            }
        }
 

            // Apply redaction to the marked bounds.
            pdfViewer.PageRedactor.ApplyRedaction();