From http://organognosi.blogspot.com.es/2011/03/how-to-connect-pdf-file-inside.html

Link one pdf to a BibDesk item's local file and URL.scpt

 --2011-03-08
--http://organognosi.blogspot.com
--the script works only for one PDF file per time

tell application "DEVONthink Pro"
    set this_item to the content record of think window 1
    set RecordLink to the reference URL of this_item
    set DevonThinkLink to RecordLink & "?page=0"
    set PdfPath to get the path of this_item
    set PdfPath to (POSIX file PdfPath) as alias
   
    tell application "BibDesk"
        activate
        set these_items to the selection of document 1
        repeat with theBook in these_items
            add DevonThinkLink to linked URL of theBook
            add PdfPath to linked files of theBook
            set currentKeywords to get keywords of theBook as string
            if (currentKeywords is not "") then
                set delimiter to ", "
            else
                set delimiter to ""
            end if
            set keywords of theBook to (keywords of theBook & delimiter & "Linked with DevonThink")
        end repeat
    end tell
   
end tell