A resident-turned-NRI is allowed to invest in the account from your non-resident ordinary (NRO) or non-resident external (NRE) account.
Earlier NRIs were not allowed to make contributions into PPF accounts opened before they became NRIs. NRIs cannot open a PPF account. However, in 2003, a notification was issued permitting NRIs to continue investing in existing PPF accounts till maturity.
At maturity you would have the option to remit the proceeds in your country of residence. You do not have the option of extension post your mandatory 15 years’ lock-in period. In case, post maturity you leave the account unattended, it will be considered as “extended without contribution”.
While interest earned on PPF is exempt in India, the tax treatment for you would be based on your country of residence and will be taxable accordingly.