Page Authority is Moz metrics, that predicts how well a specific page will rank on search engines. It is based off data from the Mozscape web index and includes link counts, MozRank, MozTrust, and dozens of other factors.
As PA consists of many metrics it's rather difficult to influence directly on it. The best way to influence this metric is to improve your overall SEO. In particular, you should focus on your link profile—which influences MozRank and MozTrust—by getting more links from other well-linked-to pages.