To enable previews in both the folder and in files, open Dolphin's settings > Configure Dolphin > General tab > Previews. In there, select "Folders" as well as whichever file type you wish to view previews for.
In the image below, I have Dolphin opened in dual-pane mode.
In the left window, I've boxed the PDFs folder.
In the right window, you can see previews of individual files.
The window on top is Dolphin's settings to enable the previews you want. Both FoldersandPostscript, PDF and DVI files are selected.
Note that there are sliders at the bottom of each pane for you to adjust the icon size for optimal viewing.
To enable previews in both the folder and in files, open Dolphin's settings > Configure Dolphin > General tab > Previews. In there, select "Folders" as well as whichever file type you wish to view previews for.
In the image below, I have Dolphin opened in dual-pane mode.