Are you positive this is occurring at download, or are you using IE or another browser that allows you to run the program as soon as its done downloading?
I have never seen UAC complain at the time anything is downloaded. It virtually always comes up at install time. Those can appear to be one and the same if you're using the settings that run as soon as download is complete. I have UAC fully on and an Unchecky download, strictly download, triggers no warning at all.
You can turn UAC "down" or "off" following Dick's instructions, but I used to hate UAC, and turned it off, but it has saved me a couple of times from unintentional installation (caused by finger twitch creating a double click when a single was intended).
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story