I have the same problem. In Zotero 4 Standalone, connector in Chrome and Firefox worked just fine. After upgrade to Zotero 5, I cannot download ScienceDirect pdf's neither with Chrome nor Firefox.
PDF download problem is reported very often. But it is something different from praveens1 problem. On another side, it is necessary to provide more info; only statement the problem is not enough for solving it.
https://flint.lithics.eu/zotero.log