Go to the section: “recherche” (~seach). There you can enter “mots du titre” (~words from the title), “auteur” (~author), “sujet” (~subject), “recherche libre” (~free text search).
When you find a title you want to investigate further, click on it. A PDF opens with the first page of the work chosen. If you go back to your browser, you’ll find a page with these options
“Notice” (~basic information about the text)
“Pagination” (~an overview of the work’s pages, click and PDFs will open)
“Texte seul” (~I’m not sure what that does…)
“Plein écran” (~full screen mode)
“Télécharger” (~download as I. said: “ok” gets you all the pages)
“Réproduire” (~requests for reproduction)