Is there any literature or well known document left written in Old French?

There is La Chanson de Roland (link). It exists in multiple MSS. An excerpt:

Carles li reis, nostre emper[er]e magnes
Set anz tuz pleins ad estet en Espaigne:
Tresqu'en la mer cunquist la tere altaigne.
N'i ad castel ki devant lui remaigne;
Mur ne citet n'i est remes a fraindre,
Fors Sarraguce, ki est en une muntaigne.
Li reis Marsilie la tient, ki Deu nen aimet;
Mahumet sert e Apollin recleimet:
Nes poet guarder que mals ne l'i ateignet.

It was composed towards the end of the Old French period. For some French that is older, you can do to the Strasbourg Oaths (part of which are in Romance) in the 9th century:

Pro Deo amur et pro Christian poblo et nostro commun salvament, d'ist di in avant, in quant Deus savir et podir me dunat, si salvarai eo cist meon fradre Karlo et in ajudha et in cadhuna cosa, si cum om per dreit son fradra salvar dift, in o quid il me altresi fazet, et ab Ludher nul plaid numquam prindrai, qui, meon vol, cist meon fradre Karle in damno sit.

(English: For the love of God and for Christendom and our common salvation, from this day onwards, as God will give me the wisdom and power, I shall protect this brother of mine Charles, with aid or anything else, as one ought to protect one's brother, so that he may do the same for me, and I shall never knowingly make any covenant with Lothair that would harm this brother of mine Charles.)

There is also some Old High German in there:

In godes minna ind in thes christiânes folches ind unsêr bêdhero gehaltnissî, fon thesemo dage frammordes, sô fram sô mir got gewizci indi mahd furgibit, sô haldih thesan mînan bruodher, sôso man mit rehtu sînan bruodher scal, in thiu thaz er mig sô sama duo, indi mit Ludheren in nohheiniu thing ne gegango, the mînan willon imo ce scadhen werdhên.

(English: For the love of God and Christendom and the salvation of us both, from this day on, as God will give me the wisdom and power, I shall protect this brother of mine, as one ought to protect one's brother, so that he may do the same for me, and I shall never go along with Lothair in anything that, by my will, would harm him [Louis].)

(Link)

It so happened today I was skimming through an historical grammar of French. It's online, but might not be available outside the US (link).

Ceci n'est pas un seing.