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hola!claude - another possibility is to keep the secondary PDF (the original I would guess) elsewhere in your file system, e.g. a folder called libraryname_originalPDFs.
Then put a link to the file in the built in Comments column, or a long_text custom column. To add such a link click this

in the Comments Editor toolbar. You can use the Browse button in the subsequent dialogue to navigate to the file.
You'll then be able to click on the Link in the Book Details panel, and the PDF Reader that your OS knows about should kick into action.
If you put the links into a custom column, then you'll probably want to add the column to the list in the
Preferences->Look and Feel->Book Details tab.
BR