PPF

As an NRI, you can not open a new PPF account. But if you had already opened your PPF account when you were an Indian resident, you can continue investing in it until its maturity. You can deposit up to ₹1.5 lakh per year, and withdrawals are allowed as per PPF rules.
And once your PPF account completes 15 years, you cannot extend it further. Extensions beyond 15 years are not allowed for NRIs.

It’s great and important to diversify your investment for retirement. You can consider:

PPF: gives you guaranteed returns and helps you in tax benefits. Debt Funds: considered as low-risk funds, gives you a steady return. ELSS Mutual Funds: gives you a tax rebate with a potential growth rate.

If you are still not able to understand, it’s better to consult a financial advisor for a personalized plan.