I use Zotero in conjunction with Papership (http://www.papershipapp.com) on an iPad. Papership syncs with WebDAV storage, which means that, after syncing, downloaded PDFs are available on your iPad for reading offline. Any annotations are pushed back and available in the Zotero client. It's a great client and really stable. In conjunction with the Apple Pencil, it's a really nice system.
I'd advise an iPad Pro 12.9" if you do a lot of reading. The extra size means that you spend less time zooming in and out, and an app like Liquid Text (http://liquidtext.net) comes into its own for bringing text and figures into the same region of the screen and reduce the amount of time you spend flipping between the two.