1. Open any PDF and wait for the program to load.
2. Click "Edit" at the top.
3. Click 'Preferences' (At the bottom of the menu).
4. Click the 'General' tab from the menu on the left (Usually third from the bottom). There will now be an option to @Enable PDF Thumbnail previews in WIndows Explorer. Check this box.
5. Wait for the extension to install.
6. Restart your PC. Your PDF icons will now appear as previews.