Govt of India has made it mandatory to link Aadhaar with PAN. This rule specifically applies for application for new PAN number. This has been done to check the tax evasion by using multiple PAN cards.
For people already holding PAN and Aadhaar which is still unlinked, a later date will be notified by the Govt.
Linking of Aadhaar with PAN is unavoidable.
Here is a link to the article on how to link Aadhaar with PAN.
Incase of above link not working, here is an alternate viewer.