If you need the content for research, request a specific page or topic from a university library’s interlibrary loan system—they may have access to facsimiles or exhibit catalogs. For casual learning, explore the Codex Arundel (British Library, free online) as a similar Leonardo notebook with open access.
It’s important to clarify upfront: The manuscript is owned by Bill Gates, who purchased it at auction in 1994 for $30.8 million. It is occasionally exhibited (e.g., at art museums or the Seattle Art Museum), but the full work is not in the public domain. The Codex Leicester Pdf-