Relativity has an annoying habit of appending numbers to the beginning of PDFs that you download from a workspace. So, files can be named with either a 1, 2, or 3 digit number followed by an underscore, and the document identifier or field selected to name the file. E.g.:
Using Bulk Rename Utility you can use this regular expression search to remove the numbering, so the files are only named with the Bates numbers:
In the replace box simply enter:
The search for the numbers with the underscore are separated in parentheses from the second part also in parentheses which searches for any number of characters.