'Get the file name
Dim filePath As String = args.GetString("txtFileName").Trim()
If filePath = String.Empty Then
   Return "Please enter a valid file name."
End If
If Not File.Exists(filePath) Then
  Return "The specified PDF file does not exist."
End If

'Get the from page index and page count
Dim fromPage As Integer = -1
Dim pageCount As Integer = -1
Try
   fromPage = Integer.Parse(args.GetString("txtFrom"), CultureInfo.InvariantCulture)
   pageCount = Integer.Parse(args.GetString("txtCount"), CultureInfo.InvariantCulture)
Catch
   Return "Either the from page index or the page count is invalid."
End Try

'Load the PDF file
Dim doc As New PdfDocument(filePath)

'Check page index range
If (fromPage < 0) OrElse (fromPage >= doc.Pages.Count) Then
   Return String.Format("From page index must be between 0 and {0}.", doc.Pages.Count - 1)
End If

'If page count is invalid, then extract
'till the last page
If (pageCount < 0) OrElse (fromPage + pageCount > doc.Pages.Count) Then
   pageCount = -1
End If

'Create a new PdfDocument object that only
'contains the pages to be extracted
Dim doc2 As PdfDocument = doc.Clone(fromPage, pageCount)

'Save the new PdfDocument into a file. This
'file will only contain the specified pages
doc2.Save(outputFileName)