I heard that Russian Orthodox Christians are considered heretics by Catholics. Are there any official Catholic doctrines, documents or quotes that deny or confirm this statement?
The primary difference between the Eastern Orthodox churches and the Catholic Church is the refusal of the former to acknowledge the primacy of the Pope. This is a discussion that dates back well over a thousand years. There are some groups of former national Orthodox churches (the Ruthenian church for example) which have in the last several centuries chosen to acknowledge the Pope as head of the Church; these have been re-admitted into the Catholic Church and are known as the Eastern Catholic Churches.
Given that the main difference is the refusal to be subject to the Pope, the Orthodox Churches are formally considered to be in a state of schism, not heresy:
Heresy is the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Supreme Pontiff or of communion with the members of the Church subject to him.
(Code of Canon Law, Canon 751)
just as the loss of charity is the road to the loss of faith, according to 1 Tim. 1:6: "From which things," i.e. charity and the like, "some going astray, are turned aside into vain babbling," so too, schism is the road to heresy. Wherefore Jerome adds (In Ep. ad Tit. iii, 10) that "at the outset it is possible, in a certain respect, to find a difference between schism and heresy: yet there is no schism that does not devise some heresy for itself, that it may appear to have had a reason for separating from the Church."