how to increase DA and PA ?

DA won't drop after you stop buying links - that's for sure. I've came across several totally abandoned (last post dated 2006-2011) websites with DA 80++.

It takes much time to increase the DA and PA for the new domain.. You have to do more and should wait for a while.. As it is new domain you are in need to spend your money and time in it.. To avoid this risky, it's better if you buy the high DA,PA domains that are expired.. It will be easy for you to move further.. Built links and make big diversity without footprints... It will help you drastically..

Good contents and Back links will do.

To increase your domain authority (DA) you can simply do these two things :
1) Get Good backlinks on high DA websites.
2) Get rid of bad links.