The Kindle has 6 font sizes. To check that they are large enough take these
screenshots and print them on paper at 167 dpi (3.6" by 4.8"). This is very close to what the Kindle screen looks like - except that the background is gray. The background does not effect readability much, particularly at the large font sizes.
If the font is large enough, then the Kindle meets all your needs. You would type comments using the (small) keyboard.
The other option with note taking is the iLiad, which has a larger screen (particularly good for large fonts) and you would take notes on PDFs only using the stylus. Handwriting with the stylus might be easier than using the Kindle's small keyboard.