I noticed the last time I watched Back To The Future (1st one) that the flyer when Doc is holding it does NOT have the message from Jennifer on it.
Depending on your version of the movie format, the relevant places to look might be different.
Jennifer writes on the flyer at: 11mins, 10seconds.
Marty looks at what Jennifer wrote at: 11mins, 22seconds.
After Marty travels to the past, and is talking with Doc:
Marty pulls out the flyer at: 54mins, 13seconds.
Doc is looking at the paper, with us able to see the back starting at: 54mins, 20 seconds.