Another vote for Inkscape here - amazing tool. I probably use less than 1% of it's functionality, but it is far and away the best PDF editor I've come across (

Another vote for Inkscape here - amazing tool. I probably use less than 1% of it's functionality, but it is far and away the best PDF editor I've come across (I work with a lot of design drawings that started life as DWG/CAD files, are exported to PDF and sent around for proofing/review and need to have changes marked up on them).