Do you need to annotate things like PDFs or take notes?

If you already have a laptop, I recommend the drawing tablet with pen for annotating PDFs and taking notes with pen tablet.  such as wacom, huion and xp-pen.

A good size screen will help with writing papers and online assignments. In addition to the app flexibility and multitasking, the file management of Windows and mac OS is helpful, too. 

In addition, Most drawing tablets now come with android mobile support so you can definitely annotate with it.

If you don't have a laptop and want the Portability, I recommend you get a standalone pen tablet, like ipad, samsung galaxy tab, and microsoft surface.

They have a nice screen, good battery life, and the Pen is pretty responsive for handwriting notes. 

 One affordable options would be samsung galaxy tab s6 lite, since they both come with pen included.