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 a great application to read and view any PDF file.
Skim is free and open-source. It is under active development by a small group of volunteer contributors and more are always welcome.
See the Skim web page for download links and release notes.
The current release of Skim requires Mac OS X version 10.6 or higher. The latest version running on Mac OS X version 10.5.x is Skim version 1.3.22, but Skim version 1.3.7 and higher seem to run into a system bug on PPC. The latest version running on Mac OS X version 10.4.x is Skim version 1.2.7.
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