As per a Supreme Court judgment passed on 9 June 2017, if you have Aadhaar, it is mandatory to link it to PAN and mention the same in your tax returns. However, if you don’t have Aadhaar, you can e-file tax returns without the same till 30 June 2017.
The Income Tax Department has made it easy for taxpayers to link their PAN with Aadhaar with just a 2 step process which does not even require to login or register at the e- filing website. This facility can be used by anyone to link their Aadhaar with PAN.
WebSite Without Login/Registration Method
Step 1 : Just go to This Link or you can directly tap on this website https://www.incometaxindiaefiling.gov.in/ and click on Link Aadhaar
Step 2 : Provide PAN, Aadhaar no. and Enter the name (avoid spelling mistakes) and submit. After verification from UIDAI which is the government website for Aadhaar, the linking will be confirmed.
WebSite Without Login/Registration Method
Step 1.
First register yourself at the income tax e filing portal, if you are not already registered.https://incometaxindiaefiling.gov.in/
Step 2.
Log in to the e-Filing portal of the Income Tax Department by entering the log-in ID, password and date of birth
Step 3.
After login just take your mouse pointer to on "Profile Setting" and a link will appear with name "Link aadhaar" just click on it.
Step 4.
Details such as name, date of birth and gender will already be mentioned as per the details submitted at the time of registration. Verify the details on screen with the ones mentioned on your Aadhaar card.
Step 5.
If the details match, enter your Aadhaar card number and captcha code and click on the “Link Aadhaar” button.
Step 6.
A message will inform you that your Aadhaar card has been successfully linked to your PAN card.

SMS Method
Send SMS to 567678 or 56161 from your registered mobile number in the following format:
UIDPAN<SPACE><12 digit Aadhaar><Space><10 digit PAN>
Example: UIDPAN 123456789123 BGPLQ1234Y