Dim openPicker As New FileOpenPicker()
openPicker.FileTypeFilter.Add(".pdf")
Dim file As StorageFile = Await openPicker.PickSingleFileAsync()
If file IsNot Nothing Then
    Dim stream As Stream = Await file.OpenStreamForReadAsync()
    pdfViewer.LoadDocument(stream)
End If

FileOpenPicker openPicker = new FileOpenPicker();
openPicker.FileTypeFilter.Add(".pdf");
StorageFile file = await openPicker.PickSingleFileAsync();
if (file != null)
{
Stream stream = await file.OpenStreamForReadAsync();
    pdfViewer.LoadDocument(stream);
}