Hypertext Markup Language (HTML) and Portable Document Format (PDF) came into existence 15 to 20 years ago as alternatives to paper articles. Both have their place in publishing; however, HTML has increasingly become the standard for online use as it is more in tune with developments in the research process.
Google Scholar, one of the largest A&I services in existence, indexes almost any journal and likely accounts for more than half of all referrals to online journals. Google Scholar is also very important as its large open index of scholarly articles is easily accessible for most readers. It also does an excellent job of finding multiple versions of scholarly articles and theses.