Title. Every so often when I open dndbeyond, there is an HTML file immediately downloaded to my PC with a randomized name, sometimes multiple files at once. Please explain.

Are you running any extensions that might cause this behavior?

What browser and OS are you using?

This is the first time I have ever heard of this happening.

Technically speaking every website in existence operates this way. The browser downloads files to read them and make the page for you and removes the files when not needed, determined by page code and your cache settings. Absolutely everything that uses the internet for anything does this. It downloads the data and then uses the data to build the page, image, audio, video, etc for you to interact with. It's not limited to internet either - radio and phone calls work similarly : the source converts the audio into data which then transmitted as a signals out - your phone or radio receives the data signal, then interprets that data into audio. This is an oversimplification but you get the idea.

So either you're not understanding how websites work, or how your devices uses websites (as they can all do things a bit differently) or you have something like an extension interfering with the process in some way. Or you might have malware.

We'll need to know more (maybe with screenshots if able?) about what you're experiencing, the devices, OS and browser you're using, etc. 

the browser is Firefox, the OS is Windows 10 and the only extensions I have are a dark theme, adblocker and beyond20

 Not sure if this is considered a necro or not, but I have started experiencing the same issue as the OP with the same exact setup. Windows 10, Firefox, and every time I come to dndbeyond it instantly downloads and saves multiple HTML files that cannot be opened in any way.

I am having the same issue on my laptop. Windows 10, Firefox, and same extensions minus the dark theme.

Scanned it with antivirus and it doesn't appear to be malicious. Just very weird.