These days, that's a distinction without a difference. All websites that couldn't usefully be delivered as PDFs are applications by definition.
Just as most new web sites are not startup companies, most existing web sites are not apps.
The distinction and difference is that a software application helps a user perform manipulation or transformation of data as useful work.
Most websites, despite simple interactivity (e.g. search), are still published as "content" for consumption within a content access application, not for manipulating work|play|creative output.