Disney Canon -
Unless I'm gravely mistaken, Luke uses a Stormtrooper blaster he picked up while rescuing Leia on board Death Star.
He shoots several stormtroopers on DS (the clearest instance I recall is, after Ben Kenobi dies, he flies into rage, and while everyone runs into Falcon, kills a couple of stormtroopers left inside the blast doors by himself).
The explanation I heard (no Disney canon backup for it) was that Stormtrooper armor could only dissipate energy from glancing shots and protected from projectiles, but NOT from direct blaster hits.
EU/Legends canon -
"Our body armor isn’t able to withstand direct blaster shots, but it is designed to survive the most extreme conditions." (source)
If this is too Twittery of a source, we can do an official C-canon: "The New Essential Guide to Weapons and Technology (2004)" says pretty much the same thing: it protects against glancing blows but not direct blaster fire.
An interesting note is that direct blaster fire usually does not KILL stormtroopers, just disables them for a while. So, Lucas/EU invented Doom's RESPAWN functionality :)