Anyone may download individual pages from public domain material in the HathiTrust. Downloading entire volumes either in PDF or text-only is a benefit of membership in the HathiTrust. Be sure you've logged in using the yellow "LOG IN" button in the upper right corner of the main screen.
Once in the full view of a resource the options to download appear in the left column under "Get this book". If logged in, the ability to download the whole book will appear along with downloading a page or pages. The file can be then be transferred onto a reading device or other computer.
Downloading a range of pages is also restricted to those logged in with a membership. To select a range of pages, click in the upper corner of the book page on the first page. It will become highlighted in orange. Go to the last page wanted and click in the upper corner of that page. The "Get this Book" options will adjust to show the option to download the number of pages selected.