Can I still buy the season pass for that game?
You will need to check the store for an updated answer, but at the time of writing, yes.
How does the season pass works?
The season pass is intended for gamers who buy the game when it was first released. Over the "season" of the game's release cycle owners of the pass will receive DLC at no extra cost. Its like buying all the content up front but before it is actually released.
Now that all the DLC is available, the season pass is effectively a bundle for all the game's DLC.
Note that this is not always the case for other games. Sometimes the season pass doesn't include ALL DLC.
If I buy it, will I get the DLC files with the trophies?
Yes. You will get the Clash in the Clouds, Burial at Sea Episode 1 and Burial at Sea Episode 2 DLC packs.
You will get extras too (weapons, skins etc) not related to trophies. You can find the exact contents on the store or official game's site.