"This page? Yes, I have read it."
Translation:Cette page ? Oui, je l'ai lue.
Because some verbs require agreement not with the subject of the sentence, but with the direct object. The majority of French verbs are conjugated with avoir in the compound tenses and do not agree with their subjects. However, avoir verbs require agreement with their direct objects or direct object pronouns when these precede the verb. (There is no agreement when the direct object follows the verb or with an indirect object.)
In the given sentence it has been qualified as feminine (cette page) and, represents a direct object pronoun that precedes the verb, therefore the past participle must agree. Hence Je l'ai lue.