The files are "ready to download" or "ready to be downloaded"?

This is tricky.

There is an obvious difference between:

I can have tea ready in 15 minutes. Are you ready to eat?

and

The stew is ready to eat.

Eat has obviously transitive, elliptically transitive (with implied object) (though some would say intransitive), and middle-verb (This curry eats well - compare This Merlot drinks well) usages. Though I think that in *The stew is ready to eat we have an ergative rather than a middle usage (The door shut suddenly vs The door shuts easily).

So, in the two examples above, arguably there are two different meanings for (to) eat:

_to consume

_to be consumed (used only in certain constructions).

The first meaning allows: Are you ready to eat?

The second allows: Is the curry ready to eat? While a paraphrase, from the first meaning of eat, using the passive, is Is the curry ready to be eaten?

Obviously, with some ready + to-infinitive constructions, there is only one possibility:

He is ready to come.

*He is ready to be comed / come. Come doesn't passivise, neither is it ergative.

You hesitate to accept the use of the term "download" in the common examples:

The files are ready to download. 

Because you feel that the word is only a transitive verb.

Ex. download the files, download music, download the software etc.

But in fact "download" is a verb that can either be transitive or intransitive. This means that you can use it even without an object.

So:

The files are ready to download. 

is shorter and perfectly natural.

Ex. Anyone who has waited for a webpage to download can appreciate the limitations of data transfer.

The OED records download as a transitive verb only, but, as the OP has noticed, it is now used intransitively as well. We can say ‘I have just downloaded some files, but we can also say ‘The files are downloading’. A verb such as this which allows the object in a transitive clause to become the subject in an intransitive clause is known as an ergative verb. Launch and dispatch are two more verbs which seem increasingly to allow both constructions.

Careful writers will want to judge how the intransitive use of verbs which have been mostly transitive in the past will be received. Where there is doubt, they might be best advised to be conservative in the matter. In the OP's example, that would mean preferring 'ready to be downloaded', although in that particular case it has to be said that the readers would probably already be familiar with the intransitive use.