Here are some sources for downloading free French textbooks and reference materials:
Gutenberg.org: Gutenberg.org is a non-profit organization that offers free access to thousands of classic French literature texts and historical documents.
Bibliotheque Nationale de France (BnF): The BnF, France's national library, provides access to a vast collection of French-language e-books, including novels, poetry, and non-fiction.
Gallica: Gallica is a digital library managed by the BnF, which offers free access to a wide variety of French-language texts, such as books, journals, manuscripts, and maps.
Wikisource: Wikisource is a collaborative project hosted by Wikimedia Foundation that contains a collection of free texts in various languages. It has a good collection of French-language textbooks and other educational materials.
Open Culture: Open Culture is a website dedicated to providing access to free cultural resources, including a collection of French textbooks and other educational materials.
L'Encyclopedie Libre: L'Encyclopedie Libre is a free online encyclopedia in French, which also offers a collection of reference materials and textbooks.
Free French Audiobooks:
LibriVox: LibriVox is a volunteer-driven project that offers free public-domain audiobooks in a variety of languages, including a large collection of French titles.
Bibliotheca Sonore Romand: Bibliotheca Sonore Romand is an online library that provides free access to audiobooks in French.
France Culture: France Culture, a French public radio station, offers a collection of audio programs, including podcasts, which can be useful for learning French or staying updated on current events in France.
TV5Monde: TV5Monde, a French international television network, offers a collection of educational videos and language courses in French.
Online French Courses:
Duolingo: Duolingo is a free language learning app that offers a comprehensive course in French.
Memrise: Memrise is another free language learning app with user-generated courses for learning French.
Frenchpod101: FrenchPod101 offers a variety of free podcasts and video lessons for learning French.
Radio Lingua Network: Radio Lingua Network provides free audio courses for learning French, which can be useful for improving listening and comprehension skills.
Remember to check the copyright status of the materials before downloading and using them for personal or educational purposes.