Some annotation features offered by AnnotationUI objects may be incompatible with particular image formats. A translucent RectangleAnnotation embedded in a PDF, for instance, renders as an opaque annotation in most PDF viewing applications.

We can sometimes get around limitations like this by substituting incompatible annotations with annotations that support more of the desired features. The code below demonstrates how to convert all the translucent RectangleAnnotations in a LayerCollection into a suitable PdfMarkupAnnotation with similar properties.

public static AnnotationDataCollection ConvertForPdfExport(LayerCollection origLayers)
{
    AnnotationDataCollection annotationData = new AnnotationDataCollection();

    foreach (LayerAnnotation layer in origLayers)
    {
        LayerAnnotation convertedLayer = new LayerAnnotation();
        foreach (AnnotationUI anno in layer.Items)
        {
            if (anno is RectangleAnnotation)
            {
                RectangleAnnotation rectangleAnno = anno as RectangleAnnotation;
                if (rectangleAnno != null && rectangleAnno.Translucent)
                {
                    PdfMarkupAnnotation replacementAnno = new PdfMarkupAnnotation(null, PdfMarkupType.Highlight);
                    replacementAnno.Location = new PointF(0,0);
                    replacementAnno.Size = rectangleAnno.Size;
                    replacementAnno.Fill = rectangleAnno.Fill;
                    replacementAnno.Quadrilaterals.Add(BoundingQuad(rectangleAnno));

                    convertedLayer.Items.Add(replacementAnno);
                }
                else
                {
                    convertedLayer.Items.Add(anno);
                }
            }
        }

        annotationData.Add(convertedLayer.Data);
    }
    return annotationData;
}

private static QuadrilateralF BoundingQuad(AnnotationUI anno)
{
    PointF ul = new PointF(anno.Bounds.X, anno.Bounds.Y);
    PointF ur = new PointF(anno.Bounds.X + anno.Bounds.Width, anno.Bounds.Y);
    PointF ll = new PointF(anno.Bounds.X, anno.Bounds.Y - anno.Bounds.Height);
    PointF lr = new PointF(anno.Bounds.X + anno.Bounds.Width,
            anno.Bounds.Y - anno.Bounds.Height);

    QuadrilateralF quad = new QuadrilateralF(ll, lr, ul, ur);
    return quad;
}

The AnnotationDataCollection returned by the ConvertForPdfExport method can be used as an argument to a PdfAnnotationDataExporter's ExportOver method, which will embed the annotations into an existing PDF. Now any annotations that started out as RectangleAnnotations with transparency will render with transparency in the PDF itself.

Take a look at this other helpful article on getting transparent annotations to render with transparency. The approach described there involves burning the annotation to the actual image, so the pixels to render the annotation become an inseparable part of the image. When working with formats that don't support embedded annotations burning may be a good option.

Original Article:
Q10334 - HOWTO: Transparent Highlight Annotations in PDF