"to" between substantive and infinitive

Solution 1:

In a comment, Professor^{Answers are competitive and I have an unfair advantage} Lawler answered:

This is what's known as a Relative Infinitive. Relative clauses are tensed clauses normally (the man who came to dinner; the chair (that/which) he sat in), but infinitives may also occur.

Relative infinitives have a number of peculiarities:

• they always imply some deontic modal like should, so the example means This is the file that one should download;
• the antecedent can be either the subject or the object of the infinitive (the man to talk to; the man to do the job);
• no relative pronoun, except when pied-piping: the man with whom to speak, but *the man who(m)/that to watch.