MainWindow.xaml

<Window x:Class="Custom_Stamp.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:Custom_Stamp" xmlns:syncfusion="clr-namespace:Syncfusion.Windows.PdfViewer;assembly=Syncfusion.PdfViewer.WPF"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <!-- Button in the first row -->
        <Button Grid.Row="0" Content="Button" Width="100" Height="30" Margin="10" Click="Button_Click"/>
        <syncfusion:PdfViewerControl Grid.Row="1" x:Name="pdfViewer"></syncfusion:PdfViewerControl>
    </Grid>
</Window>