The HathTrust Digital Library is a digital repository of materials from member libraries and beyond. Generally, it's not possible to view an in-copyright title in full. You will see HT links to Princeton scanned materials via PUL catalog records. Otherwise, "public domain works deposited in HathiTrust without restrictions (such as works scanned by Internet Archive or other organizations) and works made available by rights holders under Creative Commons licenses are available for full download." Login via PUL for optimal use.
From the Public Collections or My Collections tabs, login using your Princeton ID to see public PDFs.