Alternative
Another possibility: the future PCs don't come back, they can only send messages back. Also, to prevent paradox, they aren't allowed to say that they are in the future. Unfortunately, memory being unreliable, they send back several messages, but the messages "miss" the PCs by a few moments to a few hours.
People at the locations PCs have just left find the notes, and assume the PCs read and rejected the pleading that the must not perpetrate this great evil. These people hire "heroes" (or become them) to stop the PCs "nefarious deeds".
How do the enemy know the PCs secrets / weaknesses? Well, the notes each contain information about the PCs to help convince them that they were sent by a friendly party.
Perhaps, not only are the enemy questing to stop the PCs, they want to gather all the notes, so that they are better able to overcome the (possibly more powerful) PCs. This means sticking close to the PCs and/or retracing their steps. The enemy don't tend to keep the notes on them (travel is hard on documents, and they want the others of their group to have access to the information if they are destroyed by the PCs), so the PCs can only rely on 2nd hand knowledge as filtered through the people who attacked them.
. . . . . . . -- Eric
Another possibility: the future PCs don't come back, they can only send messages back. Also, to prevent paradox, they aren't allowed to say that they are in the future. Unfortunately, memory being unreliable, they send back several messages, but the messages "miss" the PCs by a few moments to a few hours.
People at the locations PCs have just left find the notes, and assume the PCs read and rejected the pleading that the must not perpetrate this great evil. These people hire "heroes" (or become them) to stop the PCs "nefarious deeds".
How do the enemy know the PCs secrets / weaknesses? Well, the notes each contain information about the PCs to help convince them that they were sent by a friendly party.

. . . . . . . -- Eric