What is the difference between freeing up space and ″deep clean″?

Free up space: The original, uncropped pictures and exported PDF files saved in the [CamScanner] folder will be deleted from your device;

Deep clean: Besides the images above, cropped and enhanced scans (with high resolution) will also be cleared (preview images will be kept).

When you need to read them in the CS app, just open that doc to download the images.