What does this mean? Did the PDF standard include all of PostScript as part of it (I thought PDF was more like a subset?) or did it allow (but no longer allows) embedding PostScript objects?
Yes, parts (or all) of a PDF page could be expressed in PostScript as special objects. The separate (from PostScript) regular drawing primitives of PDF are inspired by PostScript, but those only form a sub-language within the PDF format, which adds enveloping structures and many other features.
