They were really designed to show the same output on the screen and printer.
Even if you are using the same word processor software and file format, a document can look vastly different when you send it to someone else who doesn’t have the same screen resolution or the same fonts installed.
PDF started as just a print preview for the postscript printer language. They should have just stopped there instead of trying to make it do all sorts of other shit that can open security holes.
The constant parade of file formats drove popularity, but it was really about being the only popular format to look the same.