As the oxidation state of metal in metal oxide increases its acidic nature
 

Acidic character increases with increase in oxidation state because the tendency to gain electrons increases with the increase in oxidation state.

Compounds in which the central atom is present in a high oxidation state like Mn₂O₇.
Mn is present in the +7 oxidation state, the compound is electron-deficient and has a tendency to accept electrons. Thus, it is acidic in nature.