As mentioned by Leanne, I explained what I do with URL field in that thread.
I don’t think Attachment field can be modified in the way you think, but URL field accepts file path as well.
One additional note about URL is, you can use ctrl+G to open the linked file, but you can’t open multiple files at the same time. If you have multiple lines in the URL field, you need to click on them one by one.
In any case, are you going to attach 1000 PDF files one by one? How these files are named?
As I explained in the other thread, I save PDF files with my unique ID number (pubmed ID), which is available in Endnote’s entry as well. This makes “independent” link between PDF files saved in separate location and each entry in the Endnote. If your PDF files are named, like author, title, theme, etc, you would not be able to automate the link.