Onyxboox has a variety of eInk readers.
To have a good reading and avoid problems with "pdf not fit to screen" a big screen like OnyxBoox Max or
OnyxBoox Prometeus
OnyxBoox MAX
Display: E Ink Mobius, 13,3", touch, resolution 1200 × 1600 dots, 16 levels of grey
"Its large and comfortable for your eyes display, .. optimal for reading PDF and DjVu files"
Lighting: No
Processor: 1 GHz
RAM: 1 GB
Internal memory: 16 GB
Audio: Stereo output, 3,5 mm, Speaker, Microphone
Expansion slot: For micro SD cards
USB interface: USB 2.0
Supported file formats
Text: TXT, HTML, RTF, FB2, FB2.zip, DOC, DOCX, PRC, MOBI, CHM, PDB, DOC, EPUB
Graphic: JPG, PNG, GIF, BMP
Others: PDF, DjVu, MP3
Wi-Fi Wi-Fi IEEE 802.11 b/g/n
Bluetooth 4.0
Battery: Polymer Li-on, 4100 mAh
Operating system: Android 4.0.4
Size 325 × 237 × 7,5 mm
Weight: 480 gr


PROMETEUS
Display: 9,7", E Ink Pearl, 16 levels of grey, 825 × 1200 dpi, touch
Lighting: MOON Light
Processor: Freescale i.MX6, 1 GHz
RAM: 1 GB
Internal memory: 16 GB
Expansion slot : For SD/MMC/SDHC cards
USB interface: microUSB
Supported file formats:
Text: TXT, HTML, RTF, FB2, FB2.zip, DOC, DOCX, PRC, MOBI, CHM, PDB, DOC, EPUB
Graphic: JPG, PNG, GIF, BMP
Others: PDF, DjVu
Wireless interface: Wi-Fi IEEE 802.11 b/g/n
Bluetooth 4.0
Battery Li-on, 3000 mAh
Operating system Android 4.0.4
Size 258,2 × 177,3 × 9,5 mm
Color: Black. Weight: 450 gr

MartinK said about Onyx Boox M92, that is hackable.


Although an e-reader can read pdf files, the reading it is not so comfort as on .epub files. The pdf was designed to deliver the document in the exact aspect as was created (same fonts, size, margins, etc). That exacteness doesn't fit well on 6", 5" screens.
In the other hand, epup is designed precisely to do that. So you can choose the size, font, interlined, etc acording to your preference and your e-reader device. Even the build-in dictionary works on epub files (not in pdf).
An EPUB file is a ZIP archive that contains, in effect, a website—including HTML files, images, CSS style sheets, and other assets. It also contains metadata.