Whom should I send the email to? The Principal Investigator or the postdoc who have done the work?

If the paper lists a corresponding author, then that is the person to try. Otherwise, I would just email all the authors and let them decide which one will respond. I don't think it's impolite and I'd rather email too many than too few authors (and risk getting no response).

First, it's not entirely clear to me that the PI is listed as an author on the paper. (I assume the PI probably is, but if not, then it would odd to email the PI for this.)

If you have had prior contact with either the postdoc or the PI, such that they might recognize your name in their inbox, then that might be a reason to start the contact there first. If your name is familiar to them, they are more likely to see your email and remember to deal with it.

With those caveats, I agree with Thomas's advice to contact the corresponding author (if any) or all authors at once.

Who should I send the email to?

Is the PI is listed as an author?

• Yes: Write them both, as 'To' addressees.
• No: Post-Doc as 'To', and 'CC' the PI if s/he will want to be aware of the interaction with you.

Also, if somebody else is the corresponding author, they should be a 'To' addressee as well.

Is it considered polite to send the email to the postdoc and CC his PI?

Not if the PI is listed as an author. Better to mail them both and let them sort out who wants to answer you. ... that is, assuming PI and the postdoc are on good terms with each other.