If you want to have previews of PDF files in Dolphin (useful for E-books collections), follow these simple steps:
1. Open Synaptic Package Manager.
2. Search for and install kde-graphics-thumbnailers.
3. Open Dolphin file manager.
4. Go to Settings -> Configure Dolphin -> General -> Previews and enable "Show previews for Postscript, PDF and DVI files"
(Note: you may need to edit the limit of file size under "Do not display previews for..." if you have large PDF files)
5. Restart Dolphin.
6. Select "Preview" from the Dolphin toolbar for the folder containing your PDF files.