Plagiarism detection has been considered as a classification problem which can be approximated with intrinsic strategies, considering self-based information from a given document, and external strategies, considering comparison techniques between a suspicious document and different sources. In this work, both intrinsic and external approaches for plagiarism detection are presented. First, the main contribution for intrinsic plagiarism detection is associated to the outlier detection approach for detecting changes in the author's style. Then, the main contribution for the proposed external plagiarism detection is the space reduction technique to reduce the complexity of this plagiarism detection task. Results shows that our approach is highly competitive with respect to the leading research teams in plagiarism detection.