public partial class Form1 : RadForm
{
    public Form1()
    {
        InitializeComponent();
        this.radPdfViewer1.LoadDocument("../../example.pdf");
        this.radPdfViewer1.MouseClick += RadPdfViewer1_MouseClick;
    }

    private void RadPdfViewer1_MouseClick(object sender, MouseEventArgs e)
    {
        var textBoxControlElement = GetTextBoxControlElement(this.radPdfViewer1);
        if (textBoxControlElement != null)
        {
            if (textBoxControlElement.ForeColor != Color.White)
            {
                textBoxControlElement.ForeColor = Color.White;
            }
        }
    }
    private RadTextBoxControlElement GetTextBoxControlElement(MyPDFViewer myPDFViewer)
    {
        RadElement editor = null;

        foreach (RadFixedPageElement visualPage in myPDFViewer.PdfViewerElement.ViewElement.Children)
        {
            if (visualPage.Children.Count >= 1)
            {
                editor = visualPage.Children.FirstOrDefault();
            }
        }
        return editor as RadTextBoxControlElement;
    }
}