They are the same. "Durga Saptashati" refers to the 700 verses; "Devi Mahatmya" means "The Glory of the Goddess." The Kannada PDF will often use the title ದುರ್ಗಾ ಸಪ್ತಶತೀ (Durga Saptashati).
: A dedicated portal for stotras that includes the Kannada script for the Durga Saptashati. durga saptashati kannada pdf
The Goddess understands your heart. However, if you want to avoid errors, listen to an audio recitation (YouTube has many Kannada artists like "Bangalore Sisters") while following along in your Kannada PDF. They are the same
Visit the official Gita Press website → "Downloads" or "Grantha" section. if you want to avoid errors