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katiusciad - Do you have the book in any format other than PDF - eg a word processing format, plain text or html. PDF is the worst of all formats to convert to EPUB (or any other ebook format).
If not you probably need to edit the EPUB itself - here's a link to the calibre book editor manual ==>>
http://manual.calibre-ebook.com/edit.html You'll see there's a section in there re ToC's
It looks a bit daunting at first, but if all you want to do is get a ToC then it should be fairly straightforward
Ask any questions in the Editor subforum and you'll get plenty of help
BR