It's not a usability decision — it's just one feature that hasn't yet been implemented [1]. Updating metadata is more complicated than creating a new item, because you have to deal with the existing metadata somehow. (Mendeley just overwrites the existing item, often with incorrect data, which we don't consider acceptable.)

Disclosure: Zotero developer

[1] https://github.com/zotero/zotero/issues/1515

I really love the "duplicate finder" and its ability to pick and choose metadata to keep. Perhaps just create a duplicate item and deal with merge conflicts later?

I'm not sure what feature exactly you're missing, but are you aware of the "Rename file from parent metadata" option you get when right clicking on a PDF in the list? Perhaps that does what you need.

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 look up metadata if you add a PDF to your library

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

I see! Yes, that's not great.