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.
System Requirements: Mac OSX 10.6 or higher.
Download Skim from SourceForge
Skim: Manual
Skim: Frequently Asked Questions
Skim: Support
Referenced from: Skim-app.sourceforge.net.