Defining individual styles with the 'id' and 'class' attributes.

1. Main Item 1: text files
   • Sub Item 1:1
   • Sub Item 1:2
2. Main Item 2: text files
   • Sub Item 2:1
   • Sub Item 2:2
3. Main Item 3: pdf files
   • Sub Item 3:1
   • Sub Item 3:2
   • Sub Item 3:3