@BetterRed: thanks for your tips! I have tried again to save metadata in calibre with "edit metadata" into an unprotected .pdf but it doesn't work for me. I have checked it with PDFMtEd (uses ExifTool) and PDF Exchange or you mean something else with "calibre Embed Metadata tool"? How did you do it?
I won't even try with Mendeley because it couldn't get metadata like Zotero.
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