title_body: How to create clickable table of contents in a PDF?
upvoted_answer: Well, you could always buy a copy of Adobe Acrobat, which is actually designed solely for the purpose of creating and editing PDF files.
  
  Or you could import the file into Calibre, convert it to ePub format, edit the ePub to add the table of contents, then convert it back to PDF.
  
  There are other free tools for working with PDF directly. If you do a Google search, you would find this page which lists several free tools for editing or modifying PDF in various ways.