5 gallons of heartburn?

Just moved my first cider out of the primary, into he secondary along with some PS. Pulled some test samples (of course) and it was good, but very acidic. So much so that only a few sips really burned. Not an alcohol burn, definitely acidic. The recipe was just apple juice, water and sugar. Safale 05 yeast.

I back sweetened with a syrup of 2 cups water, 2 cups brown sugar, 1 tsp cinnamon. It helped a little, but it still burns.

Is there anything I can do to help before I bottle? RDWHAC? This is my first cider so you may tell me to chill out and let it mellow. I thought that I read somewhere that cider is good to drink as soon as it is done?

Does it have a vinegar smell? There's not much you can do if it has gone vinegary. If it smells fine but tastes acidic a MLF will help, you need to keep the cider at or above 65F for a MLF, and no camden tablets. It can happen naturally or you might need to buy a culture. You can deacidify but it will affect the taste and you need to check the pH first.

Sometimes all the cider needs is time. Keep it in secondary for 60 days at least, then bottle it if it's completely clear. After 3 months in the bottle, some of that tart "bite" might mellow a bit. After two years, it should be great.

Whoever told you that you can drink it as soon as it fermented is either a liar or someone who drinks bad cider.

No vinegar smell at all. Excuse my ignorance, but what is MLF?

MLF is Malolactic fermentation. You'll find plenty of info on the google. Personally I wouldn't drink dry cider that hasn't been through MLF. Often the aging that Yooper is talking about will induce MLF naturally.