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There's nothing important in that file, so I don't think you made a mistake deleting this file. Anyway, if you want to restore it, you need a Time Machine backup.
If you haven't made a Time Machine backup before, you can't recover that file without reinstalling OS X, but as I said, you don't need the file
Create a new user account in the Users & Groups system preferences, log into this account, and move the About Downloads PDF file from its downloads folder to the Public directory in your user account. Then log out and back into your user account where you will find the PDF file, and you can then delete the newly created user account.