Depending on how the schematic is rendered, it is often editable as a PDF. I use Adobe Illustrator to edit PDF files including schematics. Of course this is a very expensive program if you obtain it legally. But it does work for this purpose. So it is not correct to state that editing a PDF rendering of a schematic is useless. But I never do this for my designs as I have the original CAD schematics. But I have done it for schematic PDFs from obsolete programs. Even if the PDF is rendered as BMP and not vectors, you can cover errant sections, and draw over it with AIs tools if you just need minor changes. Then, of course, AI can save it as an editable PDF.
That being said, it does not allow you to update net lists or use library parts (unless you take the time to make AI library parts).
I use AI for all kinds of electronic purposes such as assembly prints, PCB fab drawings, etc. In some ways it is better than traditional CAD programs for this as you can easily include photos, text, drawings, etc, and even import DXFs from CAD programs.
I suppose there is a possibility that this works for me because I do all my design on a Mac, and AI on Windows machines may not work the same for editing PDFs.