I've worked on PDF editing library long enough to have a very good understanding how PDFs can break. And also good enough understanding why they're designed as they are - there are some rather forward thinking approaches the authors took in 1.0 and 1.2 spec standards.

Editing actual page contents is hard but the format itself wasn't designed with live editing in mind. Instead, they made a tradeoff to render the document consistently on all devices which has made the format so popular.