Padma Purana is one of the eighteen principle Puranas, as explained in Shrimad-Bhagavatam (12.7.25):

"The eighteen major Puranas are the Brahma, Padma, Vishnu, Shiva, Linga, Garuda, Narada, Bhagavata, Agni, Skanda, Bhavishya, Brahma-vaivarta, Markendeya, Vamana, Varaha, Matsya, Kurma and Brahmanda Puranas."

In a conversation with his disciples, Shrila Prabhuapada tells about the importance of the Padma Purana:

Srila Prabhupada: "So, there are different kinds of Puranas, eighteen Puranas …but, t he sattvika,-puranas (those in goodness)-they are meant for immediately worshiping the Supreme Personality of Godhead……So Padma Purana is one of the Puranas in the mode of goodness, passion, and ignorance, and to reclaim all these conditioned souls, there are eighteen Puranas. Six Puranas are meant for those in the mode of goodness, six for those in the mode of passion, and six for those in the mode of ignorance. The Padma Purana is written for those in the mode of goodness."