Placing documents into the public domain

DJ Bernstein:

Most rights can be voluntarily abandoned (“waived”) by the owner of the rights. Legislators can go to extra effort to create rights that can’t be abandoned, but usually they don’t do this.

In particular, you can voluntarily abandon your United States copyrights:

It is well settled that rights gained under the Copyright Act may be abandoned. But abandonment of a right must be manifested by some overt act indicating an intention to abandon that right. See Hampton v. Paramount Pictures Corp., 279 F.2d 100, 104 (9th Cir. 1960).