The "ars praedicandi" was literally the "art of preaching" - an elaborate theory which set down the rules and guidelines - both moral and technical - by which all preachers must abide. Chaucer was familiar with the "ars", possibly only because of the many sermons he would have invariably listened to. The moral aspect of the "ars" is that which the Pardoner is guilty of perverting. He is a "vicious" man, and a man guilty of avarice, but the "ars" states that a preacher should not preach for personal gain, but to spread the word of God, and...
with the "ars praedicandi" then, is quite far, but only when one just takes into account his dexterity in and competence for the work. When one considers his motivation for preaching, his devotion to doctrine seems less enthusiastic, to put it mildly. The Pardoner, as a literary figure, is a paradox - he preaches out of avarice, to avarice, and he can tell a moral tale even though he is a "ful vicious man". His preaching is strengthened by the knowledge which supports it - he perverts the preacher's art, and yet is more powerful because of it.