In Zodiac, there is a 4 year timeskip where several headlines are read. They represent events in American history that (most) Americans probably recognize, with classics playing alongside (Roberta Flack - Killing me Softly, America - A Horse With No Name, The Temptations - Papa Was A Rollin' Stone, Bachman-Turner Overdrive - You Ain't Seen Nothing Yet, etc).
This is the script of the scene, along with some annotations of mine:
We today have concluded an agreement to end the war and bring peace with honor in Vietnam. - Ending of the Vietnam War
...Charles Manson for the Tate-La Bianca mass murders... - Manson family murders
Guilty in the Chowchilla kidnapping case. - School bus abduction
In the murder trial of one... - ?
Chairman Mao died today... - Mao Zedong
...an absolute pardon unto Richard Nixon... "No, I'm not a crook..." - Richard Nixon and his pardon
... Generation of hostility... - ?
...has now led to the death of 50,000 Americans and several hundred thousand Vietnamese... - Possibly the Vietnam war again?
Patricia Hearst is kidnapped by the Symbionese Liberation Army... - Patty Hearst
All of us should stand up together and say, "No more." - ?
Your Declaration of Independence spreads that... - ?
Americans admire the United Kingdom... - ?
Mr. Hoffa has disappeared. - Jimmy Hoffa
If we despise our own government... - ?
...that the police have captured a man whom they believe to be the "Son of Sam... " - David Berkowitz
As you can see, I could not identify 6 of these headlines (and there is one I'm not so sure about).
What do these titles refer to?
They most likely occurred between 1971 and 1976 (the timespan of the scene). They seem that they might be important and knowing what they are might help understand some elements of the movie.