A canonical representation means, that for any two semantically equivalent formulas $f_1$ and $f_2$ and a fixed variable order, $f_1$ and $f_2$ are represented by the same reduced and ordered BDD.

Since semantically equivalent formulas have the same set of satisfying models and a reduced and ordered BDD does not have redundancies, the representation of $f_1$ and $f_2$ are the same.