I don't know what kind of security your target server has, but in general, you can use a code pattern like this
url = "ftp://username:password@ftppath/www_logs/testfolder/test.csv"
download.file(url, destfile = "test.csv")
Obviously, this will not work if the target server has security restrictions and does not allow for password-based authentication (which is the case in most real-life scenarios), in that case you can use more specialized packages like curl or ssh