The annual report is a great and free way to research a company. The Securities Act of 1933 and Securities Exchange Act of 1934 require public companies to submit a variety of forms including the Annual Report to Stockholders (ARS) and 10-K Report.
The databases listed in this guide make it easy to find Annual Reports, and they actually get a lot of their information from sources like the Annual Report, but they are also typically available for free. If you don't have access to databases, you can still find the Annual Report (as long as it is a public company). You can always start with the company's website. Just look for an investors link.