The
E-book Reader Matrix contains a row on PDF Capabilities. For example, the DR1000S has good PDF support, but the DR800S has pedestrian PDF support and in particular no zoom at all.
In the 6" devices, the Oynx Boox 60 has good support (based on its Wacom touch pen, like the iRex devices), as does the touch-less PocketBook 301+. The Cybook Gen3 has PDF zoom, but it does not have PDF reflow.