public IReadOnlyCollection<int> GetSelectedThumbnailPageIndexes()

Public Function GetSelectedThumbnailPageIndexes As IReadOnlyCollection(Of Integer)

using System.Drawing;
using System.Drawing.Imaging;
// ...
void ExportSelectedPages(object sender, ItemClickEventArgs e){
    // Obtains the selected page indexes.
    var pages = pdfViewer1.GetSelectedThumbnailPageIndexes();
    // Saves each page from the collection to an image.
    foreach (var i in pages){
      Bitmap image = pdfViewer1.CreateBitmap(i, 1000);
      image.Save($"..\\MyBitmap{i+1}.bmp", ImageFormat.Bmp);
    }
}

Imports System.Drawing
Imports System.Drawing.Imaging
' ...
Private Sub ExportSelectedPages(ByVal sender As Object, ByVal e As ItemClickEventArgs)
    ' Obtains the selected page indexes.
    Dim pages = pdfViewer1.GetSelectedThumbnailPageIndexes()
    ' Saves each page from the collection to an image.
    For Each i In pages
      Dim image As Bitmap = pdfViewer1.CreateBitmap(i, 1000)
      image.Save($"..\MyBitmap{i+1}.bmp", ImageFormat.Bmp)
    Next i
End Sub