var provider = new Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider();
            string filePath = @"../../../InteractiveForms.pdf";
            Telerik.Windows.Documents.Fixed.Model.RadFixedDocument doc = provider.Import(File.ReadAllBytes(filePath));

            foreach (RadFixedPage page in doc.Pages)
            {
                foreach (var a in page.Annotations)
                {
                    VariableContentWidget widget = a as VariableContentWidget;
                    if (widget != null)
                    {
                        widget.TextProperties.Fill = new Telerik.Windows.Documents.Fixed.Model.ColorSpaces.RgbColor(255, 255, 255);
                       
                    }
                }
            }

            this.radPdfViewer1.Document = doc;