I know of PDFMod, which I rather like except it has one flaw, I can't seem to be able to add bookmarks before pre-existing bookmarks (see the attached image if this is unclear).
I've looked at this question: - Which programs can I use to edit PDF files?
That question does not deal specifically with bookmark editing, neither does it specify which software is freely available and which isn't.
I should add that I'm incompetent when it comes to installing software so any software that's not available in the Software Centre requires you to detail exactly how I'm meant to install it. Plus, I would specifically like a software that can handle large (like >80 MB) PDF files without lagging like crazy or closing down.