This code can be used to read bookmarks in PDF using C#. For detailed guide please check https://kb.aspose.com/pdf/net/how-to-read-bookmarks-in-pdf-using-c-sharp/

Read Bookmarks in PDF using C#.cs

using System;
// Add reference to Aspose.PDF for .NET API
// Use following namespace to read bookmarks in a PDF File
using Aspose.Pdf;

namespace ReadBookmarksInPDF
{
    class Program
    {
        static void Main(string[] args)
        {
            // Set license before reading bookmarks in PDF file
            Aspose.Pdf.License AsposePDFLicense = new Aspose.Pdf.License();
            AsposePDFLicense.SetLicense(@"c:\asposelicense\license.lic");

            // Open PDF from which you want to extract bookmarks
            Document pdfwithbookmarks = new Document("ReadBookmarks.pdf");

            // Retrieve PDF bookmarks by iterating through them
            foreach (OutlineItemCollection bookmark in pdfwithbookmarks.Outlines)
            {
                Console.WriteLine(bookmark.Title);
                Console.WriteLine(bookmark.Italic);
                Console.WriteLine(bookmark.Bold);
                Console.WriteLine(bookmark.Color);
                // Check if bookmark has any children
                if (bookmark.Count > 0)
                {
                    Console.WriteLine("Reading Child bookmarks...");
                    // If children are present then loop through them as well to extract bookmarks
                    foreach (OutlineItemCollection childBookmark in bookmark)
                    {
                        Console.WriteLine(childBookmark.Title);
                        Console.WriteLine(childBookmark.Italic);
                        Console.WriteLine(childBookmark.Bold);
                        Console.WriteLine(childBookmark.Color);
                    }
                }
            }
            Console.WriteLine("Program has finished reading PDF bookmarks in provided file.");
        }
    }
}

foreach (OutlineItemCollection childBookmark in bookmark)
The fact that bookmark is an IEnumerable is a bit counterintuitive, even if that's the way it's internally stored in PDF.
I would prefer having a dedicated .Children property for accessing child bookmarks, regardless of whether it is a computed property or not.