The only time the Menu says "open" instead of "open with" is when the file type is *not* ePub or pdf.
For those two you get "open with" which leads to a list of installed reading apps...
...unless your extensions.cfg (Its a plain text file) has been tweaked to block off ADE...
...and that requires direct intent...
Not sure how that could happen; the factory default is *supposed* to be ADE not FBReader, but to verify/fix it see this thread:
https://www.mobileread.com/forums/showthread.php?t=67149
If you did receive a PB360 with an altered extensions.cfg file please report it, okay?
A while back, GreenReader's web site was reported as advertising the PB360 as lacking DRM support, which isn't supposed to correct. But if ADE is missing or blocked by an altered extensions.cfg file...
Curioser and curioser...