Have you been reading your PDF textbooks on your Kindle up to now? If so, and are happy enough with how they look and you appreciate that PDF's on 6" ereaders are far from ideal (a tablet is always the recommendation for someone wanting to read PDFs) your best option is probably a Sony, as while it's far from perfect for PDF's, it's better than the Kobo & Nook.
Either the PRS-T1 or the T2, the latter is slightly newer and faster, has a nicer matte finish and slightly improved battery life, but doesn't have audio capabilities, and has annoying sticky-up buttons). The T2 new costs £119 but you could buy either refurbished
direct from Sony for £80-85).
You could perhaps ask in the Sony forum for feedback on which of the two is better for PDF's.