Use the CLICsearch Online Catalog to find church documents. Here are some suggestions:
Items by individuals are listed under the authors, e.g., try: Writings and Pius XI.
Documents from the Congregations, Curia, committees and other official bodies are listed under the name of the group (sometimes in Latin), e.g., Try searching for Congregation of the Doctrine of Faith.
Many items written by popes are assigned a special heading, Papal Documents