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 (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall