Oleg asked: "Shouldn't it be "Il les a tous lu" instead of "Il les a tous lus"? If not, why?"

Bonjour Oleg !

"Il les a tous luS" is actually correct, because it follows a (very annoying!) rule of agreement with the auxiliary "avoir".
You will agree the past participle with the direct object of the verb IF this one is placed BEFORE the verb... which happens when using direct object pronouns as in here.
Here is a link to the related lesson:
Special cases when the past participle agrees (in number & gender) when used with 'avoir' in the compound past in French (Le Passé Composé)

I hope that's helpful!
À bientôt !

Hi Joan, 

In

Tous is an adjective