What readers annotate PDFs better than the Preview app?
Skim is a pdf reader specialy designed to annotate pdf.

Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file.
Stop printing and start skimming.
Explore the links to the left to investigate Skim and consider helping out with the project.


  Features:
  
  
  Viewing PDFs
  Adding and editing notes
  Highlighting important text, including one-swipe highlight modes
  Making "snapshots" for easy reference
  Navigation using table of contents or thumbnails, with visual history
  View all your notes and highlights
  Convenient reading in full screen
  Giving powerful presentations, with built-in transitions
  Handy preview of internal links
  Focus using a reading bar
  Magnification tool
  Smart cropping tools
  Extensive AppleScript support
  Bookmarks
  Saving passwords in Keychain
  Export notes as text
  Automatic download of remote PDFs
  Support for Apple Remote Control
  Interaction with LaTeX, SyncTeX, and PDFSync
  Integration with BibDesk and other third party applications
  Spotlight support
  Highly customizable
  And much more...