Trying to use the raspberry pi to control an Arduino Uno and I'm tyring to ensure nothing breaks when connecting them. I know that the Arduino outputs at 5V while the pi inputs at 3.3V. This requires resistors to decrease the voltage.
What about the other way around? I would like the pi to output a digital signal to the Arduino to have a function run. Common sense suggests that this shouldn't be a problem but I want to make sure that is the case.
The Arduino GPIOs will be fine. Some Arduinos are 3V3, most are 5V. Either way their GPIO will not be damaged by a 3V3 signal from the Pi.
The answer depends on what you mean by "connecting them".
Normally I use a 3.3V Pro Mini which can be directly connected.
Strictly the Pi GPIO "HIGH" signal is marginal when connected to 5V logic, but usually works. You can use a voltage divider on Pi inputs.
If you need bi-directional use a level converter (which can always be used for one-way).
