For Indian customers, PayPal has different cashback offers in place at the moment. Firstly, I would recommend checking your eligibility for the specific offer both on Paypals website and on the merchant's website. Generally, the offer is valid only for first-time users’ first transaction on PayPal and can be availed once per PayPal user. Please note that if you have already used PayPal in the past, even with a different account, you would not be eligible for the offer. Also, the cashback voucher will be in the form a discount for your next purchase and no credit will be issued to your balance.
If the offer is specific for one merchant, though, Terms and Conditions may be different! I would recommend reaching out to them directly too. Here is the cashback offer webpage on Paypals website where you can find more information. https://www.paypal.com/in/smarthelp/article/FAQ3976