Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF.
License: Open Source
Apps available for Mac OS X
Google Docs Viewer is a google document viewer app. Developed by Google
License: Free
Apps available for Online Discontinued
Google Docs Viewer is ideal for collaborative document editing and viewing online, whereas Skim excels in PDF annotations and offline reading, making it more suited for academic purposes.