What is the proper term for a person who uses words incorrectly, hoping to impress others?
Pretentious is a good word to describe such a person. I don't know of a word that specifically refers to using words in this way. You could say "verbally pretentious".
If the speaker were female, Mrs. Malaprop was a fictional character who used words incorrectly and the source of the word malapropism. If the speaker were male, calling him Dogberry is a less known, but parallel fictional character, who used malapropisms.
It seems that both Mrs. Malaprop and Dogberry were trying to impress their hearers, but came up with a wrong word with humorous effect.
Anyone engaging in illegal financial transactions will be caught and persecuted.
(He meant prosecuted.) -George W. Bush, Washington, D.C., Sept. 19, 2008
The reforms we seek would bring greater competition, choice, savings and inefficiencies to our health care system.
(He probably meant efficiencies. This is more of a Freudian Slip, and less a malapropism.) —Barack Obama, in remarks after a health care roundtable with physicians, nurses and health care providers, Washington, D.C., July 20, 2009