Hopefully, I can get you started on the right path, but I'm no expert.
EPUB is pretty much compatible across the board. PDB compatibility appears to be less common. According to the
matrix, the Pocketbook 360 and Onyx Boox can read PDB.
Regardless, you can always use the free and open-source Calibre software to convert files to and from EPUB/PDB.
I'm going to guess your main problem will be removing the DRM (if any). I don't know about DRM compatibility across devices, since I've made it a point never to buy books with DRM.