It does that, and it can actually look up metadata from pdfs. But: It can NOT update already existing entries from such a lookup. You have to delete the pdf and re-import. Why? No reason.
It's this sort of really bad usability decision that makes Zotero just not very good imo.
Disclosure: Zotero developer
If not, with Zotero you have a chance of getting a missing feature added. Not so with the closed-source Mendeley. Especially now they have started doing user-hostile lock-in stuff like gratuitously encrypting users' data, making it no longer accessible to the user (see downthread).
With Zotero, in principle you could hack together something using `bash` and `sqlite3` that batch-updated documents; you may not even need to look at the source code to the official app. It's ad-hoc munging like that that Mendeley has recently gone out of its way to prevent, stopping people from working effectively with their own data.
Zotero can not update metadata for a PDF you already have in your library
Hence, adding an existing library means you have to do it manually for each pdf This is bad design, because the functionality exists, but it's implemented in a way that makes everything complicated for no reason at all.
Why disable the "lookup metadata" button for a pdf which is already in a library? WHYYYYY