Interestingly, I had a conversation with a student about precisely this issue. Of course, overtly at least, this excellent dystopian novel is written in very accessible and simple English. The level of vocabulary is not too elevated and there is a sense in which Orwell is trying to communicate his story simply and effectively.
However, and this is what my student discovered, there are parts of this novel that are a lot more complicated and that are written with a much higher level of sophistication and complexity. The main example of this is the lengthy addition of the book within a book, when Winston Smith reads The Theory and Practice of Oligarchical Collectivism by Emmanuel Goldstein, which really discusses quite complex topics such as world politics, history, policy and strategies for maintaining control. These chapters are very different from the normal style of the novel, and are definitely more of a challenge to read.
In spite of this, I would still argue that this is an unpretentious work. There is a definite sense in which parts are more readable than others, but apart from this, the novel does not come across as pretentious in my opinion.