from ironpdf import *

# Create a new PDF or edit an existing document.
pdf = PdfDocument.FromFile("existing.pdf")

# Add bookmark
pdf.Bookmarks.AddBookMarkAtEnd("Author's Note", 2)
pdf.Bookmarks.AddBookMarkAtEnd("Table of Contents", 3)

# Store new bookmark in a variable to add nested bookmarks to
summaryBookmark = pdf.Bookmarks.AddBookMarkAtEnd("Summary", 17)

# Add a sub-bookmark within the summary
conclusionBookmark = summaryBookmark.Children.AddBookMarkAtStart("Conclusion", 18)

# Add another bookmark to end of highest-level bookmark list
pdf.Bookmarks.AddBookMarkAtEnd("References", 20)

pdf.SaveAs("existing.pdf")