In some circumstances, exporting the image will work. The exported image will usually be lower-quality than the original, as this is how smart previews are stored by Lightroom.

However, sometimes, the images will not be exportable even with a smart preview. In these situations, you should go find your smart previews.

Note: This will only work if you still have the folder containing your Lightroom catalog files.

Where Smart Previews are Stored

The smart previews are stored in the Lightroom Catalog Smart Previews.lrdata folder in your Lightroom catalog folder. For more information on where Lightroom catalog folders are stored, please visit this Adobe FAQ.

Inside the folder, there are other folders, from 0—F. Inside those folders are even more folders. Inside these deep folders are the Lightroom smart previews. These smart previews are stored as DNG files. These DNG files are lower resolution and are smaller (both in file size and in pixel dimension size). On a PC, DNG files can be opened by Microsoft Photos, although by default, they will open in Snip & Sketch.