Hi,
PDF interactive documents were not intended to be the alternative of web based applications. There will be legality involved when you intend to convert paper based to electronic version. And, what ever the user typed in a pdf, digitally signed should be a legal proof even after 100 years like how a normal hard copy will be a legal proof. This cannot be achieved with normal web based screens, as the softwares change, Operating systems change, from time to time and it may get tough to find whether the website based proof can be trustable or not.
But, PDFs got approved universally and once a pdf document is preserved as an archive file, no matter what change you see in the technology after decades, the archive file will look exactly the same as you see today.
Hope your question is answered.
Regards,
Chaitanya