The Larger Lesson Here Is...
Adobe adopted the idea that PDF was a container for offline web information so they wanted to be able to bundle up all the features of the web (viruses included but not intended) into a compact portable format. Knowing all the investment they had in dynamic content from stuff like Macromedia it was obvious they wanted to be able to deliver animated flash presentations and shit like that in a PDF, they probably saw it as a PowerPoint killer. Well, they succeeded at bundling up all the (destructive) power of the web into an easily portable format for exchange. They succeeded so well they have now become one of the primary conduits for malware.