If you're using Windows 7:
For Pdf file - Install foxit PDF Reader (it's free and fast) and toggle the preview pane in windows explorer.
For Video files - Install gom player, it's also a very able player that smooths video files, once a codec pack is installed, such as k-lite codec pack. (there are tones of skins available - even a mac quick look skin) In the preferences the key closest to your enter key to close the program, for me it's - ' - assign it to {Basic Exit} This way you can easily preview a file, in high quality and easily close the file.
For Image files install picasa photo viewer. It has a very transparent aero design which gives the impression of previewing the file.
Make sure installed programs are saved as default player.
Ifran is cool, but so ugly piece of software :) And it's only a image viewer. – crea7or – 2012-03-25T13:28:53.850
@crea7or actually, Irfanview supports many different file formats, and even includes a media player inside the application itself (audio/video formats are at the very bottom of the list). Note that in order for Irfanview to support some of these formats, you may have to install an appropriate plugin first.
– Breakthrough – 2013-07-31T17:40:34.373