PIC24 Open-Drain Config for 5V PWM Operation
Depends often on the pin.  THIS DEVICE, for example, has some pins that are 5V tolerant.  The Electrical Characteristics on page 283 of that datasheet show the 5V tolerant pins can go to 5.6V when Vdd >= 3V.  Otherwise, you can go to Vdd +0.3V on a non five volt tolerant pin.  

This may well impact your choice of pin for PWM.  You should be able to use a similar table in your device's data sheet to figure this out on your own.  Usually, in the package diagrams (like on page 8) the 5V tolerant pins are grayed in.

And yes, you should probably handle this as an open drain with a pull up resistor to 5V.