Use PageAreaRedaction Leave feedback

            using (Redactor redactor = new Redactor("Sample.pdf"))
            {
                Regex rx = new Regex("urna");
                ReplacementOptions optionsText = new ReplacementOptions("[redarea]");
                optionsText.Filters = new RedactionFilter[] {
                    new PageRangeFilter(PageSeekOrigin.End, 0, 1), // last page
                    new PageAreaFilter(new System.Drawing.Point(300, 0), new System.Drawing.Size(300, 840)) // right half of the page 300x840
                };
                RegionReplacementOptions optionsImg = new RegionReplacementOptions(System.Drawing.Color.Chocolate, new System.Drawing.Size(100, 100));
                RedactorChangeLog result = redactor.Apply(new PageAreaRedaction(rx, optionsText, optionsImg));
                if (result.Status != RedactionStatus.Failed)
                {
                    redactor.Save();
                };
            }