I’m not sure if they were ever designed to be immutable, but that’s what a lot of people use it for because it’s harder to edit them. But there are programs that can edit PDFs. The main issue is I’m not aware of any free ones, and a lot of the alternatives don’t work as well as Adobe Acrobat which I hate! It’s always annoying at work when someone gets sent a document that they’re expected to edit and they don’t have an Acrobat license!