If by pdu you mean protocol data unit(what i get when I google "pdu"), then thats a protocol not a character set. If you mean ETSI GSM 03.38(some character set i get when I google "pdu to ascii") then here is a list of pdu characters to their corresponding unicode characters. Then it is a simple matter to convert from unicode to ascii. Remember... google is your friend.
Please try this online PDU converters.
// They are not very efficient in conversion but worth trying as they need no installation...
in this source you find simple coder/decoder of PDU/ASCII
download this component named (aljanna component)
install it in Delphi 6 or 7 or above
transfort your data from ASCII to PDU by the component
then send the SMS by your mobile,