If I kill an opponent's Guile while I control a Planar Void will they get to shuffle it back into their library?
It depends on who controlled which card and whose turn it is.

Both cards trigger on the same event, namely Guile going to the graveyard. Therefore the triggered abilities go on the stack at the same time. When that happens, the active player (the player whose turn it is) starts putting his or her triggered abilities on the stack, then the other players do the same in turn order.


  603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities he or she controls on the stack in any order he or she chooses. (See rule 101.4.) Then the game once again checks for and resolves state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.


So there are 6 scenarios:

You control Guile and Planar Void: 

You choose the order of the triggers, so you decide whether Guile goes to exile or the library.

Your opponent controls Guile and Planar Void: 

Your opponent chooses.

You control Guile, your opponent controls Planar Void, it is your turn: 

Guile is exiled.

You control Guile, your opponent controls Planar Void, it is your opponent's turn: 

Guile is shuffled into the library.

You control Planar Void, your opponent controls Guile, it is your turn: 

Guile is shuffled into the library.

You control Planar Void, your opponent controls Guile, it is your opponent's turn: 

Guile is exiled.