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Whether you are downloading ebooks, gadget manuals, research papers or your monthly bank statements, chances are that all these documents are available as Adobe PDF files. That's because PDF is probably the only format that can retain the original formatting and can be read across all platforms or even on the web if you don't want to install any desktop software for reading PDFs.

Mashable's tools for school. I use some of these already, but I'd like to see some more.