Examples:

• He is at a school. (We don't know which school.)
• He is at the school. (We know which school.)
• He is at school. (He is a teacher or student and is teaching or learning.)

• I buy the paper every day. (Newspaper)
• The professor presented a paper on recent findings about cancer cells. (A piece of writing)
• The children are learning the art of folding paper into decorative objects. (Material)

Examples:

(1) When we use two or more adjectives to describe the same person or thing, we use the article only before the first adjective.

• He talked to a tall and blonde woman.
  (He talked to a [one] woman who was tall and blonde.)

(2) When we use two or more adjectives to refer to more than one person or thing, we use the article before each adjective.

• He talked to a tall and a blonde woman. 
  (He talked to two women, one of whom was tall and the other was a blonde.