He is not the actual author of even papal decrees, as He is with all the words of Scripture.
Once again, trying to use one tool for every task...
The Holy Spirit inspired the Sacred Scriptures, the Holy Spirit operates through the Sacraments, including Baptism and Holy Orders, and the Holy Spirit guides the bishop of Rome when he teaches on matters of Faith and morals.
And you are bound by the teaching of Humanae Vitae. As Christ told the apostles, whatever you bind on Earth will be bound in heaven. But I would venture that the teaching found in Humanae Vitae is too hard for you. And I would further venture that you have never read it...