Decompressor

Hence it's possible that any particular file, even if it appears random, may be significantly compressed, even including the size of the decompressor.

In any case, since the compressed data can include unused "trailing bits" the decompressor must be able to determine when to stop producing output.

The decoder must have the same probability estimation the encoder used, which can either be sent in advance, derived from already transferred data or be part of the compressor and decompressor.