Here is C# code that shows how to load document from PDF file, annotate document, save annotated document to a new PDF file: 
private static void AnnotatePdfUsingTemporaryFile()
{
    // create image collection
    Vintasoft.Imaging.ImageCollection images = new Vintasoft.Imaging.ImageCollection();
    // create annotation controller associated with image collection
    Vintasoft.Imaging.Annotation.AnnotationDataController annotations = 
        new Vintasoft.Imaging.Annotation.AnnotationDataController(images);
    // load PDF file into collection
    images.Add("sourcePdfDocument.pdf");

    // get the annotation collection for selected image
    Vintasoft.Imaging.Annotation.AnnotationDataCollection imageAnnotations = annotations[images.Count - 1];
    // create new annotation
    Vintasoft.Imaging.Annotation.RectangleAnnotationData anno = 
        new Vintasoft.Imaging.Annotation.RectangleAnnotationData();
    anno.Size = new System.Drawing.SizeF(300, 300);
    anno.FillBrush = new Vintasoft.Imaging.Annotation.AnnotationSolidBrush(System.Drawing.Color.AliceBlue);
    anno.Location = new System.Drawing.PointF(0, 0);
    // add new annotation into annotation collection
    imageAnnotations.Add(anno);

    Vintasoft.Imaging.Codecs.Encoders.PdfEncoder encoder = 
        new Vintasoft.Imaging.Codecs.Encoders.PdfEncoder();
    // specify that annotations must be saved with image collection
    encoder.AnnotationsFormat = Vintasoft.Imaging.AnnotationsFormat.VintasoftBinary;
    // save image collection synchronously to new file
    images.SaveSync("destPdfDocument.pdf", encoder);
}