using DevExpress.Xpf.PdfViewer;
using System.Windows;
using System.Windows.Input;

namespace DeterminePageNumber {

    public partial class MainWindow : Window {

        public MainWindow() {
            InitializeComponent();
            pdfViewer.OpenDocument("..\\..\\demo.pdf");
        }

        private void pdfViewer_MouseDown(object sender, MouseButtonEventArgs e) {
            PdfHitTestResult result = pdfViewer.HitTest(e.GetPosition(pdfViewer));
            MessageBox.Show(string.Format("You clicked on page {0}", result.DocumentPosition.PageNumber));
        }
    }
}

Imports DevExpress.Xpf.PdfViewer
Imports System.Windows
Imports System.Windows.Input

Namespace DeterminePageNumber

    Partial Public Class MainWindow
        Inherits Window

        Public Sub New()
            InitializeComponent()
            pdfViewer.OpenDocument("..\..\demo.pdf")
        End Sub

        Private Sub pdfViewer_MouseDown(ByVal sender As Object, ByVal e As MouseButtonEventArgs)
            Dim result As PdfHitTestResult = pdfViewer.HitTest(e.GetPosition(pdfViewer))
            MessageBox.Show(String.Format("You clicked on page {0}", result.DocumentPosition.PageNumber))
        End Sub
    End Class
End Namespace

<Window
        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:DeterminePageNumber"
        xmlns:dxpdf="http://schemas.devexpress.com/winfx/2008/xaml/pdf" x:Class="DeterminePageNumber.MainWindow"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>

        <dxpdf:PdfViewerControl x:Name="pdfViewer" MouseDown="pdfViewer_MouseDown"/>

    </Grid>
</Window>