There are multiple resources for finding the full text of a patent using the patent number, patent title, and/or inventor. Depending on what you're looking for, you can retrieve patent full text from four different databases: Derwent Innovations Index, ESp@ceNet, the U.S. Patent and Trademark Office, and Patent Lens.
The first resource to check is Derwent Innovations Index, it contains over 11 million patents from over 40 worldwide patent offices, including full text for many. For more information, go to Using Derwent Innovations Index.
If you're looking for a European patent or a U.S. patent after the 1820s, useESp@ceNet. ESp@ceNet is run by the European Patent Office, containing full text of European and worldwide patents back to the 1970s. For more information, go to Using ESp@ceNet.
If you're looking for a U.S. patent not in ESp@ceNet, use the U.S. Patent and Trademark Office. This covers all U.S. patents from 1790 onwards. Only 10 full-text downloads a day are allowed. Full text is available in HTML format and PDF format (using the Patent Fetcher). For more information, go to Using the U.S. Patent and Trademark Office.
If you want to search the full text of a patent, use OLens. OLens searches patents from the U.S., Australia, Europe, and worldwide. Full text is available in HTML format and PDF format (for selected patents). For more information, go to Using OLens.