There's no way to serve attachment files via HTTP from Zotero at the
moment, though it's possible this could exist at some point. We've
talked about it specifically for serving images to Markdown editors,
as an alternative to embedding data URIs [1].
zotero://open-pdf works in general. In Zotero 5 it should launch the
PDF via your system-default PDF reader. In the Zotero beta, it will
open the PDF in the built-in PDF reader. If either of those isn't
working for you, post to the Zotero Forums with a Debug ID.
- Dan
[1]
https://forums.zotero.org/discussion/93521/available-for-beta-testing-markdown-export-of-notes