How to Select Text Within a Quick Look Preview Window in Mac OS X

 
If you use Quick Look to view any files that feature text—such as PDFs or Word documents—you’ll notice that you can’t click and drag to highlight text. Clicking anywhere on the Quick Look window simply moves it around. However, a secret setting will let you click and drag as usual to highlight text, and you can use the standard key combination of Command + C  to copy text . 

The Quick Look window can still be moved around the screen by clicking and dragging its title bar, as with any other program window. To activate the setting, open a Terminal window (Finder > Applications > Utilities > Terminal) and type the following:

 defaults write com.apple.finder QLEnableTextSelection -bool TRUE;killall Finder 

The changes take effect  immediately.  To deactivate the setting,  open a Terminal window and type the following: 

defaults delete com.apple.finder QLEnableTextSelection;killall Finder