<Window x:Class="YourProject.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:MaquetteTelerikPDFViewer"
        mc:Ignorable="d"
        xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"      
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <telerik:RadPdfViewer x:Name="pdfViewer"/> 
    </Grid>
</Window>

    public partial class MainWindow: Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        protected override void OnInitialized(EventArgs e)
        {
            base.OnInitialized(e);
            using(Stream stream = new FileStream(@"radExport.pdf",FileMode.Open,FileAccess.Read)){
                pdfViewer.Document = new PdfFormatProvider(stream).Import();
            }
        }
    }