Primary Sources of Vatican II
Over the last several years Matthew Hazell of the New Liturgical Movement has been digitizing the Acta Synodalia and the documents from the Antepreparatory Period and Preparatory Period. You can find a list of the volumes made available on their site here. The files were placed on archive.org, where I originally discovered them. To make downloading them easier, I placed all of the files into a Google Drive folder for each series of volumes. The folder for the Acta Synodalia is here, the Antepreparatory Period here, and the Preparatory Period here. The Acta Synodalia collection is incomplete, whereas the collections for the Antepreparatory and Preparatory Periods are complete. At some point in the future this page will be expanded.
The Virgin Appearing to St. Bernard by Pietro Perugino