The iPad is the tablet of choice by the USAF for the Electronics Publications (E-Pub) program. Goodreader is one of the installed software apps. I use a 128GB iPad Mini 4 and do not see myself running out of space anytime soon. As a media consumption device, it is first class and you are not limited to iTunes for data upload to Goodreader or Apple iBooks.
There are a number of wireless flash media drives you can stick in your desktop and then wirelessly upload PDF, EPUB, JPG, MP3, MP4 files, etc. onto the iPad. Sandisk Connect and Airstash are two that I use. I also watch YouTube TV on the iPad Mini. The iPad Mini 4 128GB is available at your local big box store in the $319 to $349 price range. You can get a kneeboard Velcro strap kit at Sporty's to wear it in flight if you wish.