Linking your PAN card with your Aadhaar before December 31 is mandatory to keep your PAN operative for tax filing and financial transactions. You can do it online via the e-filing portal, by SMS, or through PAN service centres. Match key details before submission and know what happens if you miss the deadline and answers to common queries.Linking your PAN card with your Aadhaar before December 31 is mandatory to keep your PAN operative for tax filing and financial transactions. You can do it online via the e-filing portal, by SMS, or through PAN service centres. Match key details before submission and know what happens if you miss the deadline and answers to common queries.