// Create a PdfDocument object
PdfDocument doc = new PdfDocument();           
// Load a PDF file
doc.LoadFromFile(inputFile);            
// Initialize a StringBuilder to store extracted text
StringWriter highlightedTextWriter = new StringWriter();           
// Loop through each page in the document
foreach (PdfPageBase page in doc.Pages)
{
	// Get all annotations on the page
	PdfAnnotationCollection annotations = page.Annotations;                
	// Loop through the annotations
	foreach (PdfAnnotation annotation in annotations)
	{
		// Check if the annotation is a highlight annotation
		if (annotation is PdfTextMarkupAnnotationWidget highlightAnnotation)
		{
			if (highlightAnnotation.TextMarkupAnnotationType == PdfTextMarkupAnnotationType.Highlight)
			{
				PdfBrush brush = PdfBrushes.Red;
				PdfPen pen = new PdfPen(brush, 1f);                          
				for (int i = 0;i<highlightAnnotation.QuadPoints.Length;i += 2) {
					page.Canvas.DrawLine(pen, highlightAnnotation.QuadPoints[i], highlightAnnotation.QuadPoints[i + 1]);
				}
				
			}
		}
	}
}            
doc.SaveToFile(outputFile);