You may want to take a look on this thread
https://www.mobileread.com/forums/sho...d.php?t=158397
But in short, PDF is not an ebook format so your best option is a 10 inches tablet.
Trust me, any eink device will be painfully slow and too small for most PDFs. You may also need landscape mode which take lot of current readers out of the equation.
Sony and Kindle give you basic PDF reading capabilities though.