Ever since the Internet was acknowledged as a commercial success, circa 1944-1995, it has been regarded as a near-universal online reading format: it is flexible and has a simple syntax and loose conventions, ever-allowing the expansion of its standards; therefore, it's no wonder it is being widely used and accepted since then.
As HTML is so flexible and so easy to understand, it is no wonder that HTML reporting has been created: HTML reports, combined with CSS, allow the display of your data in multiple ways, changing just a few lines of code. Don't like the fact that your tables have an orange header when they should have a blue one? No problem: just change the CSS, and done.
An HTML report, however, has many shortcomings: it is hard to print, there is no guarantee of consistency among different computers, and layout freedom is very limited compared to a PDF reporting. Long story short, HTML reporting is very good for what it is intended for, but were you to print them, you should have too many headaches.
Well, as it was briefly mentioned before, what about layout freedom? If you ever wanted your document to have an specific feel, making it more light-hearted and bubbly, serious, modern – be it as you wish – you know well what I am talking about: in HTML, fonts are restricted to a set of common widely-used fonts, as Arial, Tahoma, Verdana or Courier. Do you think you can easily use Helvetica as text easily when writing an HTML report? Think again. You can use it, that's for sure – but using them is very much of a hassle compared to PDF, and this can also bring legal issues. Let's not talk about object positioning either: using something as simple as MS Word, you can post a photo virtually anywhere in your document, something that cannot be guaranteed when you choose for HTML reporting.
So, which one should you choose? HTML reports or PDF reports? The answer is simple: choose both of them. HTML reports and PDF reports do not oppose each other: rather, they complement each other. And with a good conversion tool, you will surely generate terrific content both for online HTML reports and offline PDF reports.