Could you add the function? Thanks
Hey, Xinhongri 
Have you already tried the regular expression search?
You have to set a file directory first.
I think you can do this library specific under library>library properties>General file directory
or in general for all libraries under options>preferences>Linked files
The search syntax follows the one from the citationkeygenerator and in general is some kind of RegEx: Customize the citation key generator - JabRef
You can also try to download the pdf from the internet directly from within Jabref, of course this may not always work, if the library data is incorrect or insufficient to detect the pdf, or if the pdf is behind a paywall:
I mean when you have many pdf files then use jabref auto find which pdf files have no library data and auto add its recommend library data as endnote does.
Not you have library data to find pdf.
@xinhongri Ah okay, then maybe “Tools → Search for unlinked local files” would be the dialog you want
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No such section in Tools
Ah yes, it can be found under Lookup>Search for unlinked local fles