As a University of Notre Dame librarian, you should be able to log into HathiTrust by clicking the login button, selecting University of Notre Dame from the dropdown menu, and then logging in with your Notre Dame username and password. Once you do so, you'll be able to download the full pdf through the user interface (the full pdf download option is not available to non-logged in, non-member users) at https://hdl.handle.net/2027/uva.x000274833 . That option should be easier for retrieving the full pdf than using the Data API.
If you just need the plain text OCR for a few books, you may want to download the pdfs from the user interface and use the "Export to" feature in Adobe to save the OCR that's embedded within the pdf as a txt file.
If you need the OCR for a larger number of volumes, then you may want to consider requesting a dataset (see https://www.hathitrust.org/datasets ) or using the HathiTrust Research Center services (see https://analytics.hathitrust.org/ ). The datasets are more appropriate for thousands of volumes.
Hope that helps,
User Services Librarian, HathiTrust
University of Michigan