While writers do best to avoid using so as an adjective intensifier, it is a perfectly legitimate conjunction. (Remember FANBOYS? For, and, nor, but, or yet, so.) A conjunction joins two phrases or clauses.
I was exhausted so I had no trouble falling asleep.
We missed breakfast and lunch while waiting in the ticket line, so we are now headed to the diner to eat pancakes.
Many teachers and grammarians warn against beginning a sentence with a conjunction, since you’ll likely end up with a sentence fragment:
He was determined to beat the computer game. So Giovanni arrived late at Rachel’s Christmas dinner.
My advice is to follow this rule in business writing, but in creative writing the rare use can create impact or emphasis:
She was livid to see him stroll in the door two hours after dinner was served, with a fast food bag in his hand. So she smiled sweetly and kissed his cheek, and later let her puppy scarf the slice of pie she had saved.