ManyBooks has a number of short works. There are a good number of SF stories in their "Post-30s" genre option that are extracted from SF magazines (e.g. Astounding, ...). Their "short store" genre listing also has some good pickings.
http://manybooks.net/categories/
Also, Allegory Magazine (
http://www.allegoryezine.com/ ) publishes short fiction in simple downloadable PDF format.