As a current college student on a budget, if I need to print a section of a digital textbook out, I will ( at school because I never run out of credits with the way I print ). Otherwise, I try not to spend money on textbooks because they're all either overpriced or, in the case of the last one I bought for around $50USD, it's a year long license for the book.

As for smartphone, I have done a few classes where I had the required textbook on my phone, but I mostly just keep them on my laptop/desktop.

I do prefer something like a PDF or something similar for studying, though. I find it better for me to be able to search for something I need rather than just memorizing what page it's on, but that's down to personal preference with me.