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, Espacenet, the U.S. Patent and Trademark Office, and the 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.
If you're looking for a WIPO application, a European patent, a U.S. patent after the 1820s, or newer patents from various Asian countries including China, Japan, and Korea, use Espacenet. Espacenet is run by the European Patent Office, containing html full text of European and worldwide patents back to the 1970s and page images going back to the 1800s.
If you're looking for a U.S. patent not in Espacenet or wish to do full-text searching of US patents issued after 1976, 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).