That doesn’t necessarily mean JabRef will manage to find the same info. If all fields are blank it is likely that JabRef, for one reason or another, couldn’t read the data it needed from the PDF. Using JabRef 5.3–2021-04-19 plain drag-and-drop mostly works for the pdf files I find through google scholar, but there are ongoing efforts to make the pdf import better.
If you are only getting blank entries it might be better to get the DOI/ISBN from some other source than JabRef. I have never used Docear so I don’t know if you can export what it finds to JabRef somehow.
If there are only a few entries the web-search feature might work for you.
Most likely it means that JabRef is trying to autogenerate a filename based on the empty entry. I believe the default auto-generated filename is the BibTeX key and a shortened version of the title separated by a dash. If there’s no BibTeX key and no title →
-.pdf. This is the ‘Linked file name conventions’ in Preferences → Linked Files.
A regular drag-n-drop only links in my case, i.e. the file name does not change (JabRef 5.3 Mac OS X). Does this happen for all .pdf files? If it happens for only one file or a few, would you mind posting the names/paths of those files (to rule out a bug)?