“The Oxford 3000” (PDF listing) is a selection of the 3000 most frequently used words in English. If you know them, you can understand the meaning of more than 80% of the words in a general text.
Is there something like that for French? A simplified dictionary for students would be nice too. Because I'm in a condition in which I cannot buy any book, I would prefer to find such dictionary online or in PDF format so that I can print it.
I don't know of any resource as complete as the Oxford 3000, but lists of words sorted by frequency have been compiled by lexicologists, such as:
one published on Éduscol (direct PDF link), created by Étienne Brunet, has the most used 1500 words. It is commonly used by elementary or middle school teachers.
Wiktionary has another one, with 1750 words all linked to the relevant Wiktionary page. Unfortunately, the link to the original source (“Liste de 1750 mots connus d’un enfant de 4 ans, d’après l’académie d'Amiens”) is dead.