@davidfor: thanks for your fast reaction! I have tested your updated version of "google scholar" plugin and it works! The downside with it is that I have to enter manually at least a part of the title and and a part of the author so it can fetch metadata. With Zotero and Docear one can drag'n'drop files and with a click of a button all relevant metadata will be fetched for them. Although the success rate in my case was 30%-40% (files with correct metadata). Apart from that, Zotero was able to fetch more metadata than Calibre and Docear.
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