Documentary photography styles depend mainly on the photographer himself. The styles of documentary photography can be defined by a photographer based on his take on the subject and based on his interests that reflect on his masterpieces. The viewers can easily distinguish one's work with the other with the way they project the subject on their photos.
Different documentary photography styles can be seen on photography magazines and other print media or in photo galleries where dozens of professional and amateur photographers gather to exhibit their arts. Documentary photography style of a certain photographer can be copied, but the real meaning or definition of the photographs cannot.
Learn about documentary photography styles in galleries and other exhibits, or you can also go online to view popular documentary photography. By just exposing yourself to various documentary photography styles, you'll eventually see the differences between them and know the artists' perspective regarding their photos.