OS X Yosemite: Quick Look files and folders
Use Quick Look to view a file or folder without opening it. You can use Quick Look for items in Finder windows, on your desktop, in email messages, and other places.

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Select one or more items, then press the Space bar. Or force click an item.
A Quick Look window opens. If you selected multiple items, the first item is shown.
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In the Quick Look window, do any of the following:
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Manually enlarge the window: Drag its corners.
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See the item in full-screen view: Click the Full Screen button
on the left. To return to the standard view, click the Exit Full Screen button
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See the next or previous item: Click the arrows on the left, or press the Left and Right arrow keys. In full-screen view, you can click Play to view the items as a slideshow.
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See the items in an index sheet view: Click the Sheet View button
on the left, or press Command (⌘)-Return. -
Open the item: Click Open with [App].
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Share the item: Click the Share button
on the right.
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When you’re done, close the Quick Look window: Press the Space bar, force click again, or by click the Close button
.