You can edit your posts for up to six hours after you post them. So if you post something before you go to bed and wake up in the middle of the night thinking, "What was I thinking?!", you can edit or delete your post. You have the option of letting people know you have edited it or not. Probably NOT if it is just some little thing like changing a PDF to a PFD.

See, I have just now edited this post and it indicates that it has been edited.