This is actually an extremely interesting question. 'Weak' files that are more easily compressable than others certainly exist, but with low probability.
For example, the all-zeros file is a member of the set of all random 3 megabyte files, and it would certainly be possible to compress that, if by great good fortune you were lucky enough to receive it - albeit something that is unlikely to ever happen in the possible lifetime of the universe, given current physical theories.
Is it possible to quantify this idea of a 'weak' file more accurately?
For example, the all-zeros file is a member of the set of all random 3 megabyte files, and it would certainly be possible to compress that, if by great good fortune you were lucky enough to receive it - albeit something that is unlikely to ever happen in the possible lifetime of the universe, given current physical theories.
Is it possible to quantify this idea of a 'weak' file more accurately?