perl -Mopen=:std,:encoding(UTF-8)  -pe " y/\x{2018}\x{2019}\x{201B}\x{
+201C}\x{201D}\x{201F}/\x27\x27\x27\x22\x22\x22/; "        < input > o
+utput
[download]

or multiple files at once, creating a backup of reach

perl -i.orig  -Mopen=:std,:encoding(UTF-8)  -pe " y/\x{2018}\x{2019}\x
+{201B}\x{201C}\x{201D}\x{201F}/\x27\x27\x27\x22\x22\x22/; "        in
+1 in2 in3 in4 in5
[download]

On a side note, the above one-liner crashes with  -MO=Deparse on win32