There is no relationship between the need to save tax under 80C and investing in PPF. Many investors whose 80C are full via other means invest in PPF since it offers guaranteed tax-free returns. In general, if your monthly debt investment for your portfolio as per your goal-based investing plan exceeds ₹20,000/month, you can invest ₹12,500 in PPF which adds up to ₹150,000/year.