Transformer-based models, and especially pre-trained language models like BERT, have shown great success on a variety of Natural Language Processing and Information Retrieval tasks. However, such models have difficulties to process long documents due to the quadratic complexity of the self-attention mechanism. Recent works either truncate long documents or segment them into passages that can be treated by a standard BERT model. A hierarchical architecture, such as a transformer, can be further adopted to build a document-level representation on top of the representations of each passage. However, these approaches either lose information or have high computational complexity (and are both time and energy consuming in this latter case). We follow here a slightly different approach in which one first selects key blocks of a long document by local query-block pre-ranking, and then aggregates few blocks to form a short document that can be processed by a model such as BERT. Experiments conducted on standard Information Retrieval datasets demonstrate the effectiveness of the proposed approach.