I read this and it was one of those worthless CS papers that have a lot of verbiage but totally misses the point.

The meat is buried in the middle-end of the document:

"By now you can probably see the fundamental philosophical difference between PostScript and Interpress. Interpress takes the stance that the language system must guarantee certain useful properties, while PostScript takes the stance that the language system must provide the user with the means to achieve those properties if he wants them. With very few exceptions"

All the rest of it is sort of crap.

TL; DR: Postscript is a programming language that happens to be embedded in a printer and does printer things. Interpress is a PDL.