To reproduce:
- Load a pdf file at the constructor and try to load another pdf file on Form.Load => the result is that the second pdf is not loaded.
 public Form1()
        {
            InitializeComponent();
            radPdfViewer1.LoadDocument("..\\..\\pdfSample.pdf");

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            radPdfViewer1.LoadDocument("..\\..\\test.pdf");
        }

Workaround: unload the first loaded document prior loading the second one