According to a recent supreme court judgement on 9th 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 for FY2016-17. Linking Aadhaar and PAN can be done easily through through SMS also.
The Income Tax Department has urged taxpayers to link their Aadhaar with their PAN, using an SMS-based facility. This can be easily done by just sending an SMS to either 567678 or 56161. No Internet needed. This SMS might be charged rupees 3 by some carriers (Airtel charged me). Send SMS to 567678 or 56161 from your registered mobile number in following format: UIDPAN<12 digit Aadhaar><10 digit PAN> as seen in the example below.