Markandeya Purana is one of the most important Puranas. Chapter 81 to 93 of the Purana contains the stories associated with Navratri and Durga Puja and is referred as Durga Saptashati or Devi Mahatmyam. Now the Ved Puran website is offering the entire Markandeya Purana in Hindi online. It is not in pdf format. So you can only read it online.