I'd say the difference is similar to source code vs. compiled-binary-for-a-single-target. The webpage is the source code and can be rendered for various targets

I'd say the difference is similar to source code vs. compiled-binary-for-a-single-target. The webpage is the source code and can be rendered for various targets if it's responsive. The PDF is just how it looks on a single target, and does not include how it behaves (animations, video, hover effects, etc). So both have benefits.