Re: Sigh. Not again.
I don't know whether it's still true, but a PDF effectively used to be encapsulated PostScript, which allows very flexible device independent formatting, including embedded fonts, bitmaps and vector drawing capabilities.
When it was first deployed, it used to be set up so that documents could be immutable, i.e. not changeable by the recipient, so that you could be sure that what you saw was what the creator wanted you to see.
Of course, that did not suit everybody, so now PDFs are as editable as any other document format, and can even be used to produce forms that can be filled in and returned as a PDF.