Acidic strength of oxides of nitrogen
Compounds containing hydrogen are not only acidic but we have other concepts also. Like in your case it can be explained through Lewis acid-base concept.

Lewis concept: A base is defined as a substance which can furnish a pair of electrons to form a coordinate bond whereas an acid is a substance which can accept a pair of electron. 

For example, $\ce{NH3}$ is a Lewis base since on nitrogen we have one electron pair which easily be donated to form a coordinate bond. As for acid, Take any two oxides of nitrogen for example $\ce{NO2}$ and $\ce{N2O5}$ . First check the oxidation state of nitrogen in two cases, here it is +4 and +5 . Since a substance which can accept a pair of electrons is an acid, so the nitrogen with more positive oxidation state will relatively accept pair of electrons with more easily, therefore $\ce{N2O5}$ is a better acid then $\ce{NO2}$.