A basic Freeview box will work with any TV, portable or otherwise, as long as
(i) there's a strong enough signal from the aerial and
(ii) the TV has a Scart socket.
If your portable TV only has a built-in aerial, you won't be able to use it with a Freeview box unless you can provide a signal from another aerial. If you live really close to a TV transmitter, you might be able to get a strong enough signal by using a set-top aerial but, in general, you need a roof-top aerial to get a sufficiently strong signal for digital reception.
If your portable TV hasn't got a Scart socket, you won't be able to use one of the many cheap, basic Freeview boxes which are now available. These only have 'RF loopthrough' (which means that the 'aerial out' socket only carries the same signal which comes down your aerial).
To use a Freeview box with a TV with no Scart socket, you need to purchase a Freeview box which has a built-in 'RF modulator' (so that the signal from the 'aerial out' socket contains the decoded digital information).
Chris