QuickLook is a service built into OS X that allows you to quickly preview the contents of many document types. When browsing through your files in the Finder, to view their contents simply highlight one and then press the spacebar to open the preview window.
This service should work for many common file types, including images, movies, presentations, and common formats like Word, Text, Pages, and PDFs. However, you may occasionally encounter the problem that the system isn’t showing specific documents that it ought to be able to handle. In these cases the document may simply show as its icon with information about it (size, date opened, so on), or it may show corrupt or garbled preview information.
If this happens, then you can try several things to fix the issue. The first is to attempt a rebuild of the QuickLook database and cache, which can be done with the following commands in the OS X Terminal utility (in the /Applications/Utilities/ folder):
qlmanage -r cache
When finished with these commands, try restarting the system to see if QuickLook now works properly.
In addition, you can try removing the QuickLook caches manually, which can be done by opening the user library from the Finder’s Go menu (if it is missing from this menu, hold the Option key to reveal it). Then open the Caches folder in the window that appears and locate and remove the folders called “qlmanage” and “com.apple.QuickLookDaemon.” When finished, log out and back in to your system and try using QuickLook again.
Finally, if the problem is with a third-party QuickLook plug-in, you can try reinstalling the application that supplied it, which may fix the problem. Your installed QuickLook plugins will be located in the Macintosh HD > Library > QuickLook folder.
Keep in mind that QuickLook simply will not work with some document types, so while it’s useful for previewing common formats, if you have an obscure package then you may have to open it directly in a program that can handle it to see what it contains.