The pronunciation seems to be \pa.pɛ.tʁi, \pa.pe.tʁi\ or \pap.tʁi. Other than the third one, which is probably the least common, judging by the order, doesn't the "e" require an accent to denote that it is not a /ə/?
Traditionally there is no accent in papeterie (TLFi, Larousse) despite its different pronunciations ([papεtʀi], [-pe-], [paptʀi]). The spelling of some words was reformed in 1990 ; the accent grave was added at that time. Both spelling are accepted and correct. Furthermore, in this case you use the diacritic to reflect your actual pronunciation. Ac.9 says that the word is often pronounced like « pè » i.e. [ε]. With [paptʀi] some sound is dropped or compounded, just feels to me like a quicker pronunciation of [papetʀi], but this has no bearing on the spelling.