Linux

Unless you have explicitly digitally-signed the PDF, it's not immutable. It's maybe more-annoying to modify, but one shouldn't rely on that.

And there are ways to digitally-sign everything, though not all viewing software has incorporated signature verification.

No, it's too preserve formatting when distributed. Editing is absolutely possible, always were, it's just annoying to parse the structure when trying to preserve the format as you make changes

No, although there's probably a culture or convention around that.

Originally the idea was that it's a format which can contain fonts and other things so it will be rendered the same way on different devices even if those devices don't have those fonts installed. The only reason it's not commonly editable that I'm aware of is that it's a fairly arcane proprietary spec.

Now we have the openspec odt which can embed all the things, so pdf editing just doesn't really seem to have any support.

The established conventions around pdfs do kind of amaze me. Like contracts get emailed for printing & signing all the time. In many cases it would be trivial to edit the pdf and return your edited copy which the author is unlikely to ever read.