The problem with PDF is that it's a bag of needles disguised as a piece of paper. Most of the time people expect a PDF to be a document, not a Form, Rich Media, Contract, Javascript, or any of the other crap it can do. All that extra crap dramatically increases the attack surface area of Acrobat or any other PDF reader that supports it.

At least the PDF reader in Firefox is a Javascript App that runs in a Browser sandbox and doesn't support 99% of the crap a PDF can do.

Sure, but you can open a PDF in a new browser window. I'd rather not broaden my trusted codebase by installing another PDF reader.

OSs have this annoying habit of condensing multiple windows of a single application down to one taskbar item/dock item/whatever.

On Windows this can easily be remedied in the options accessible via the taskbar. I always turn this off and tell it to show the full window titles instead of just the icons. Windows are not browser tabs, I don't ever have enough of them open to need that stacking behaviour.

Another option for Chrome/Windows is to open a Guest window or an Incognito window, which is treated as a separate window-group.

Too late to edit: I see now that only a Guest window gets its own window-group. Incognito windows do not.

What is the difference between having multiple pdf reader windows vs multiple browser windows then? If you are on a mac: cmd+` is your friend.

I only have one PDF window open.