There is info that you have not provided, so I have tried to add some clarity (IMHO) to your first paragraph . . .
You: I make maps so I have tons of PDF files on an extra drive (that is not even on my main hard drive)
Me: OK (BTW, map making is an art - bless your care in such work)
You: ALL of them have been converted to “Brave HTML Document” , as shown in the File Type
Me: OK - but they (files in question) may not actually be HTML documents
You: When I open them they automatically open in a Brave online browser.
Me: From your writing, it sounds like a Brave Browser on a proxy machine (online the Internet to which your computer has access), that is tasked to Open HTML files on your computer - when you give the Open command on your computer.
You: Yes, I can right-click and open them via Adobe Acrobat
Me: Good to know, at least that Open a PDF file command, works
Skipping some of your info (understanding your frustration)
Maybe the PDF files are still PDF files, but some gremlin changed the filename extensions from .PDF to .HTML — and, given a possiblity that Brave Browser is somwhere assigned to open HTML files, then a Brave Browser (somewhere) did that: Opened the files.
In other words, you present a number of problems, yet you may help solve one or more, by helping Brave Support (I am not a member), by re-thinking what you wrote.
Please try to be more careful and specific.
Because Brave Support will be looking for “the thing” that detectives try to detect, in order to resolve a riddle such as you present.
Brave Support gets a lot of reports and requests every day. In order to ensure that your issues get addressed/resolved:
[They request] Please adhere to the guidelines to the best of your ability, and use the Template shown in the Reply Editor.
How to submit a Bug Report: