Simply put, to view a Chrome HTML file, you don't need to have Adobe Acrobat Reader — you just need Google Chrome.

Researchers warn that attackers are relying more on malicious HTML files in their attacks, with malicious files now accounting for half of all HTML attachments sent via email. This rate of ...

HTML attachments almost double the rate of the next type of file that was found to be malicious. For reference, here are the other types of malicious files: Text (9%) XHTML (4%) Binaries (0.3%) ...

No, this is a built-in feature of File Explorer when you download an html file. It's not a new feature either, it's been in there since at least Vista, possibly even XP.