Is it safe to eat raw, leftover spinach?

Yes. However fresh spinach should be consumed within 5-7 days of purchase. Unused portions should be thoroughly wrapped in a plastic bag or cellophane to retard browning and potential spoilage. The spinach should be stored at a temperature of 40°F or cooler.
 
Packaged spinach should be stored in an airtight bag. Consume leftovers by the "use by" date or even sooner if the storage bag isn't airtight. Packaged spinach should be stored at a temperature of 40° F or cooler.
 
Source(s):
Wikibooks.org "Cookbook: Spinach"
Colorado State University Extension "Health Benefits and Safe Handling of Salad Greens"