Public companies are required to file certain documents with the Securities and Exchange Commission (SEC). Use the SEC's EDGAR system to search by company name or ticker symbol for filings. You can filter your results to find filings of a particular form type, for example 10-K (Annual Report) or 424 (Prospectus).
Note that EDGAR contains corporate filings dating back to 1994. For earlier filings, you may make a Freedom of Information Act (FOIA) request of the SEC; fees may apply.