I'm planning to buy some Europa Universalis IV DLC's due to the steam sales. What i wanted to know is if i can buy them, and install the DLCs later when i finished my current game or will i be forced to install them the next time i launch the game ?
While the other answers about disabling are correct I can't add a comment, so I'll post this is an answer.
In the case of Europa Universalis IV, this is a non-issue as you can activate and deactivate DLC as you please from the launcher that boots when you hit play game.
You can disable DLC for a game in the game properties like this:
Yes, and you can later add it to your game (or disable if you want that for some reason) right from the game's page in your library by (un)marking the corresponding checkbox: