The problem was "have documents that look the same on any device, including printed paper and computer screens", and the approach was "PostScript does that for printers, let's simplify it and make it more universal". Both the problem it's solving and the approach were fine, maybe even great. Since then over three decades have passed, pdf has gained a plethora of features, some less well thought out than others, and real-world requirements are completely different than they were in the early 90s. If we were to invent pdf today it would likely look completely different. But it's still good enough that it's hard for a new format to offer an advantage compelling enough to replace pdf.