There's one point Glenn forgot to mention: GIF files support multiple "frames," which means they can be used for simple animation. I believe this feature is exclusive to GIF since, until Netscape supported it, no one had bothered with this rather obscure part of the GIF specification. PNG was designed before Netscape supported multi-frame GIFs, and the designers didn't see the point of supporting this completely unused feature. Pity.
Option-click to Hide Apps Quickly
This trick has been available in the Mac OS for years, but many people still don't know it. If you have too many windows cluttering up your screen, you can hide specific ones easily as you work. When you're in any application, hold down the Option key and click on another app's window, on the Dock, or in the Finder to switch to that other app and simultaneously hide all the windows in the previously current app.
better-looking. Edit your PDFs anywhere. Sign contracts, make
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Syncs via iCloud and Dropbox. <http://smle.us/tbpdfpen-ipad>