@KarthikB17219650 please clarify your question. 

Where did you see mention of PDN accounts?

It is a legacy term (Pega Developer Network)  for the current Pega Community account users set up to be part of the Pega community support infrastructure.  You use this login to access Pega's resources, including Academy, support, documentation, and community forums. It is recommended to use your organization account as the primary account and a personal account as an alternate to back up your information and prove ownership if you ever change organizations.

PDN User Account update and link to my company