PDFs are now industry standard as they can be revision controlled, digitally signed and encrypted for integrity.
The only manufacturer in the past I know of that published manuals in HTML was Airbus. This system worked well and had less overhead (ran better on a PC of limited means) than the PDF version, and could be revised, but was not secure.
In my opinion, paper is hard to beat for initial learning, while electronic versions are great for review.
Missing in most PDF manuals are the old colour coding schemes for revisions, differences and bulletins. Really made the changes stand out.