Should I use the pathfinding library Client side or server side? Why?
Client-side greatly reduces your costs. The server is doing less calculation, so you need less server power per player. Assuming you want to display the path to the client, doing the calculation server side requires you to send the whole path down in one big chunk, while doing it on the client only requires sending up the actual movements (which is about what you'd require in bandwidth anyway).
There are several downsides to doing it on the client. Their severity depends on your game. If the player is controlling squads of AI agents in PvP games (e.g., an RTS) then client-side pathing would allow clients to gain unfair advantages by modding their client. There may also be lag issues resulting in less-satisfying paths/movement in environments rich with dynamic obstacles.
You've told us nothing about your game that helps to inform the decision, but as an educated guess, I'd suggest doing pathing on the client. Just remember to always make movements validated on the server so the player can't send up illegal moves and warp though walls or such.