How to split WinForms PDF document based on each level of bookmarks?

using Syncfusion.Pdf;
using Syncfusion.Pdf.Parsing;
using Syncfusion.Pdf.Interactive;

Imports Syncfusion.Pdf
Imports Syncfusion.Pdf.Parsing
Imports Syncfusion.Pdf.Interactive

//Load the PDF document
PdfLoadedDocument lDoc = new PdfLoadedDocument(@"../../Data/BookmarksPDF.pdf");
Dictionary<PdfPageBase, int> pages = new Dictionary<PdfPageBase, int>();
 
//Maintain collection for page and its index
for (int i = 0; i < lDoc.Pages.Count; i++)
{
   PdfPageBase page = lDoc.Pages[i] as PdfPageBase;
   pages.Add(page, i);
}
 
//Get all the bookmarks
PdfBookmarkBase bookmarks = lDoc.Bookmarks;
 
//Iterate throws all the bookmarks.
for (int i = 0; i < bookmarks.Count; i++)
{
   //Get the bookmark.
   PdfBookmark bk = bookmarks[i];
   TotalBookmarks.Add(bk);
   Getbookmark(bk);
}
 
//Create a dictionary with a bookmark and split up page range
List<SplitRange> splitRanges = new List<SplitRange>();
 
GetPageofBookmarks(lDoc, splitRanges, pages);
 
//Spit the PDF based on range of bookmark
for (int i = 0; i < splitRanges.Count; i++)
{
   int startIndex = splitRanges[i].startIndex;
   int endIndex = splitRanges[i].endIndex;
 
   for (int j = 0; j < splitRanges.Count; j++)
   {
      if (splitRanges[j].startIndex > startIndex && splitRanges[j].endIndex <= endIndex)
      {
         splitRanges[i].endIndex = splitRanges[j].startIndex - 1; ;
      }
   }
   //Create a new PDF document
   PdfDocument document = new PdfDocument();
 
   //Import the pages to the new PDF document
   document.ImportPageRange(lDoc, startIndex, endIndex);
 
   //Save the document in a specified folder
   document.Save(DataPathOutput + "Output_" + i + ".pdf");
 
   //Close the document
   document.Close(true);
}
 
//Close the document
lDoc.Close(true);
splitRanges.Clear();
pages.Clear();
 
//Opens the folder where the output documents are saved
System.Diagnostics.Process.Start(DataPathOutput);

//Split bookmark with pages
private static List<SplitRange> GetPageofBookmarks(PdfLoadedDocument lDoc, List<SplitRange> splitRanges, Dictionary<PdfPageBase, int> pages)
{
   for (int i = 0; i < TotalBookmarks.Count; i++)
   {
      if (TotalBookmarks[i].Destination != null)
      {
         PdfPageBase page = TotalBookmarks[i].Destination.Page;
 
         if (pages.ContainsKey(page))
         {
            int startIndex = pages[page];
            int endIndex = startIndex;
            if (i + 1 < TotalBookmarks.Count && TotalBookmarks[i + 1].Destination != null)
            { 
               page = TotalBookmarks[i + 1].Destination.Page;
               if (pages[page] - 1 > startIndex)
                  endIndex = pages[page] - 1;
            }
            else
            {
               for (int j = 0; j < splitRanges.Count; j++)
               {
                  int checkStartIndex = splitRanges[j].startIndex;
                  int checkEndIndex = splitRanges[j].endIndex;
                  if (endIndex > checkStartIndex && endIndex < checkEndIndex)
                  {
                     endIndex = checkEndIndex;
                     break;
                  }
                  else
                     endIndex = lDoc.Pages.Count - 1;
               }
            }
            splitRanges.Add(new SplitRange() { bookmarkTitle = TotalBookmarks[i].Title,                    startIndex = startIndex, endIndex = endIndex });
         }
      }
  }
 
   return splitRanges;
}

//Get all level of bookmarks 
public static void Getbookmark(PdfBookmark bookmark)
{
   for (int i = 0; i < bookmark.Count; i++)
   {
      PdfBookmark bk = bookmark[i];
      TotalBookmarks.Add(bk);
      Getbookmark(bk);
   }
}

'Load the PDF document
Dim lDoc As PdfLoadedDocument = New PdfLoadedDocument("../../Data/BookmarksPDF.pdf")
Dim pages As Dictionary(Of PdfPageBase, Integer) = New Dictionary(Of PdfPageBase, Integer)()
'Maintain collection for page and its index
For i As Integer = 0 To lDoc.Pages.Count - 1
    Dim page As PdfPageBase = TryCast(lDoc.Pages(i), PdfPageBase)
    pages.Add(page, i)
Next
'Get all the bookmarks
Dim bookmarks As PdfBookmarkBase = lDoc.Bookmarks
'Iterate throw all the bookmarks.
For i As Integer = 0 To bookmarks.Count - 1
   'Get the bookmark.
   Dim bk As PdfBookmark = bookmarks(i)
   TotalBookmarks.Add(bk)
   Getbookmark(bk)
Next
'Create a dictionary with a bookmark and split up page range
Dim splitRanges As List(Of SplitRange) = New List(Of SplitRange)()
 
GetPageofBookmarks(lDoc, splitRanges, pages)
'Spit the PDF based on range of bookmark
For i As Integer = 0 To splitRanges.Count - 1
   Dim startIndex As Integer = splitRanges(i).startIndex
   Dim endIndex As Integer = splitRanges(i).endIndex
   For j As Integer = 0 To splitRanges.Count - 1
     If splitRanges(j).startIndex > startIndex AndAlso splitRanges(j).endIndex <= endIndex Then
     splitRanges(i).endIndex = splitRanges(j).startIndex - 1
     End If
   Next
   'Create a new PDF document
   Dim document As PdfDocument = New PdfDocument()
   'Import the pages to the new PDF document
   document.ImportPageRange(lDoc, startIndex, endIndex)
   'Save the document in a specified folder
   document.Save(DataPathOutput & "Output_" & i & ".pdf")
   'Close the document
   document.Close(True)
Next
'Close the document
lDoc.Close(True)
splitRanges.Clear()
pages.Clear()
'Opens the folder where the output documents are saved
System.Diagnostics.Process.Start(DataPathOutput)

'Split bookmark with pages 
Private Function GetPageofBookmarks(ByVal lDoc As PdfLoadedDocument, ByVal splitRanges As List(Of SplitRange), ByVal pages As Dictionary(Of PdfPageBase, Integer)) As List(Of SplitRange)
For i As Integer = 0 To TotalBookmarks.Count - 1
 
   If TotalBookmarks(i).Destination IsNot Nothing Then
      Dim page As PdfPageBase = TotalBookmarks(i).Destination.Page
 
      If pages.ContainsKey(page) Then
         Dim startIndex As Integer = pages(page)
         Dim endIndex As Integer = startIndex
 
         If i + 1 < TotalBookmarks.Count AndAlso TotalBookmarks(i + 1).Destination IsNot Nothing Then
            page = TotalBookmarks(i + 1).Destination.Page
            If pages(page) - 1 > startIndex Then endIndex = pages(page) - 1
         Else
             For j As Integer = 0 To splitRanges.Count - 1
                 Dim checkStartIndex As Integer = splitRanges(j).startIndex
                 Dim checkEndIndex As Integer = splitRanges(j).endIndex
 
                 If endIndex > checkStartIndex AndAlso endIndex < checkEndIndex Then
                    endIndex = checkEndIndex
                    Exit For
                 Else
                    endIndex = lDoc.Pages.Count - 1
                 End If
             Next
         End If
 
         splitRanges.Add(New SplitRange() With {
         .bookmarkTitle = TotalBookmarks(i).Title,
         .startIndex = startIndex,
         .endIndex = endIndex
         })
      End If
   End If
Next
 
Return splitRanges
End Function

'Get all level of bookmarks
Public Sub Getbookmark(ByVal bookmark As PdfBookmark)
   For i As Integer = 0 To bookmark.Count - 1
      Dim bk As PdfBookmark = bookmark(i)
      TotalBookmarks.Add(bk)
      Getbookmark(bk)
   Next
End Sub