Last time I was a student, I didn't need cutting-edge performance, just something that wouldn't choke on large PDFs, had good battery life, a solid keyboard, was robust enough to handle some rough use, and was easy to repair in case something happened.
I went with a used Thinkpad T480. If you get the variant with the large external battery, you get a total of 96 Wh, which gives outstanding battery life even with a less-efficient 8th-gen Intel CPU. It's also the last T-series Thinkpad with two SO-DIMM slots, though later models still have one slot. It runs Debian stable, and does so flawlessly. My only real complaints are the screen's aspect ratio (give me 16:10, or even better, 4:3) and brightness (250 nits), and the overall weight of the laptop (1.8 kg). That and I still can't find a Markdown editor that I like as much as the one I used on my decade-old Macbook.
If you just need to open PDFs, web browse and write essays, secondhand is definitely the way to go.