Technically, sharing a story file without the direct permission of the author is a breach of the copyright laws.
Morally, it's a personal choice. Many see it as sharing the good story that the author had originally allowed people to read for free.
Edit to add: In regards to my stories, I do not mind people sharing the html copy available on SOL or FS and am prepared to approve certain publications the right to print most of my stories for free. But I do not approve other websites placing copies of my stories on their sites without direct prior approval by me, nor the use of the copies available from Lulu or dpdotcom without my direct prior approval.
I know people who have no problems sharing a copy of a story that had been available for free, but will never share anything they had to pay for. It's your call.