A few years ago, Zotero actually made me content with the fact that I would need an external PDF reader. Then they unexpectedly rolled out their own, but by this point I am so happy with Okular that I have barely even tried Zotero's built-in PDF reader. Mostly because of Okular's great annotation, which Zotero then automatically synchronizes. A far cry from Mendeley's terrible walled garden.
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