Hi mapletony,
from what I read around these forums the iRex readers do the best job of displaying pdf in their natural format so not trouble at all with formulas etc. You may want to look into this thread
https://www.mobileread.com/forums/showthread.php?t=49817
or google for academic eReader and it will lead you back to relevant threads on this forum.
There are three models: The iliad has an 8" screen, zoom and annotation capabilities, the DR1000s has a 10.1" screen and also zoom and annotation capabilities but less of a battery life, and overall both systems can be buggy. The most recent DR800 also has an 8" screen a newer design, longer battery life but so far no zoom and no annotation capabilities. Though these may be added in firmware updates later on. The emphasis is on "may".
Hope this helps.