Public Domain (PD) libraries are large collections of software maintained by organisations that are willing to send you the software for a nominal fee (usually the cost of a disc and postage). This software can be freely distributed as long as it is either 'freeware' (software that can be freely distributed but the author holds the copyright), 'public domain' (software that can be freely distributed and the author has surrendered copyright), 'shareware' (software that can be used for a short time, but a registration fee is applicable after a time), or 'demo/crippleware' (restricted cut-down versions of commercial software). It is worth checking the documentation of any 'PD' software as sometimes the author has misunderstood or has made a genuine mistake in the classification of the software.