Hi,
I think you may define one main Root element and then three sub-root elements in the format (if it suit your requirements) e.g
Sections
Section 1
Sectoin 2
Section 3
Sample code: Workbook workbook = new Workbook();
Cells cells = workbook.Worksheets[0].Cells;
Aspose.Cells.Cell p = cells["A1"];
p.PutValue("Preface");
Aspose.Cells.Cell A = cells["A10"];
A.PutValue("page1");
Aspose.Cells.Cell D = cells["H15"];
D.PutValue("page1(H15)");
workbook.Worksheets.Add();
cells = workbook.Worksheets[1].Cells;
Aspose.Cells.Cell B = cells["B10"];
B.PutValue("page2");
workbook.Worksheets.Add();
cells = workbook.Worksheets[2].Cells;
Aspose.Cells.Cell C = cells["C10"];
C.PutValue("page3");
PdfBookmarkEntry pbeRoot = new PdfBookmarkEntry();
pbeRoot.Text = "Sections";
pbeRoot.Destination = p;
pbeRoot.SubEntry = new ArrayList();
PdfBookmarkEntry subPbe1 = new PdfBookmarkEntry();
subPbe1.Text = "Section 1";
subPbe1.Destination = A;
pbeRoot.SubEntry.Add(subPbe1);
PdfBookmarkEntry subPbe2 = new PdfBookmarkEntry();
subPbe2.Text = "Section 2";
subPbe2.Destination = B;
pbeRoot.SubEntry.Add(subPbe2);
PdfBookmarkEntry subPbe3 = new PdfBookmarkEntry();
subPbe3.Text = "Section 3";
subPbe3.Destination = C;
pbeRoot.SubEntry.Add(subPbe3);
workbook.SaveOptions.PdfBookmark = pbeRoot;
workbook.Save("f:\\test\\TestBookmarks.pdf");
Thank you.