Apple's Quick Look feature in OS X is a convenient way to preview a document without needing to launch any applications to do so.
Just select a document and press the spacebar, and if the system has a plug-in for handling the document type then the Quick Look server will generate and display the document preview.
Quick Look is intended to be a lightweight option, so the document preview may not be accurately formatted, but it should be enough to show you what the document contains. However, there may be times when this does not work correctly: you might see just a file icon along with some information about the document, or you may see a preview image that is garbled or disorganized to the point where you cannot make it out.
If you are not able to see a preview of the document, then the Quick Look plug-in for that format is either missing or not working properly. Apple supplies Quick Look plug-in support for common file formats such as text, media files, and Web pages, but developers can provide their own plug-ins for their specific file formats. If you are trying to preview a document for a standalone program such as Apple's iWork suite, then you may need a dedicated plug-in for that.
So, the first thing to do if you are only seeing a document icon in the preview window is to make sure the Quick Look plug-in for the document type is present. To do this, first go to the following directories on the system and make note of the Quick Look plug-ins in them:
/username/Library/QuickLook/ (if present)
By default, only the first directory listed above should have any plug-ins in it. The other directories should contain third-party plug-ins supplied by developers to support their applications. Apple's included plug-ins are the following:
- Audio.qlgenerator -- Support for audio files (MP3, M4A, WAVE, MIDI, and so on)
- Clippings.qlgenerator -- Clipboard text, image, and link clippings
- EPS.qlgenerator -- PostScript and illustrator files
- iChat.qlgenerator -- Saved iChat transcripts
- LocPDF.qlgenerator -- Localized PDF documents
- Office.qlgenerator -- Microsoft Office documents (Word, Excel, PowerPoint)
- Package.qlgenerator -- Folders of contents displayed as individual files
- PDF.qlgenerator -- Basic PDF files
- SceneKit.qlgenerator -- Lion-specific "scenekit" 3D-rendering documents
- Security.qlgenerator -- Keychains and certificates
- StandardBundles.qlgenerator -- Application and preference pane bundles
- Text.qlgenerator -- Plain and rich text documents
- Web.qlgenerator -- HTML documents
If your document type is not supported by any available plug-in, then you will need to install a plug-in that will read the document. In most cases you can contact the developer for a specific application to see if a plug-in is available for its documents, but if not then you can try finding one yourself at several Quick Look plug-in resource sites, including Quick Look Plugins, QLPlugins.com, and Mac Tricks and Tips.
If you do have a Quick Look plug-in that should be able to read your document type and you still can't create a preview of the document, or if the preview appears garbled, then it is likely that the plug-in is not loaded correctly or the Quick Look server cache is corrupted and needs to be rebuilt. First try running a permissions fix on the boot drive using Disk Utility to ensure that the Quick Look plug-ins and other files the Quick Look server needs are properly accessible, and then force Quick Look to reload the plug-ins and its cache by running the following commands in the Terminal:
qlmanage -r cache
After this is done, clear out the Quick Look configuration files by going to the /username/Library/Preferences folder (in Lion press the Option key and select "Library" from the Finder's Go menu), and then removing the files called "com.apple.quicklook.ui.helper.plist," "com.apple.quicklookconfig.plist," and "com.apple.QuickLookDaemon.plist." With these files removed and the Quick Look service reloaded, try previewing your documents again.