If I use “Find broken attachement link” from the tools menu, Endnote finds some but not all broken links.
Those it finds are labeled with a paper clip with a red cross (image 1). But there are others which show a normal pdf icon but have a zero byte pdf file and the text displayed “the following file could not be opened. This may occur for a number of reasons including but not limited to: the file is corrupt or the relative link was deleted”. In the corresponding folder is a pdf file with the correct name and a .pdf, but it is empty (zero bytes).
I sync the database between two computers with Mac OS X (El Capitan), and somehow this syncing procedure creates lots of problems, and I don’t know what the reason might be…