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You can create a formula field that shows the internal record ID of each record. All of these IDs are unique.
However, record IDs are not meaningful.
You could use a formula field that combines information the the record with an auto number field to ensure uniqueness. Note that if any of the underlying fields change values, the formula will also change value and may no longer be in sync with your external manually named file.
It is also possible to use the date/time that a record was created for uniqueness, as long as records are never created at the same time. Note that you may need to use a combination of formula to get the date/time in a format that can be used as a file name.