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Microsoft PowerPoint files (PPT files for short) are widely used today but steganographic schemes for them have rarely been reported. Through detailed analysis, we discover that the PPT file update mechanism, which is employed by Microsoft PowerPoint Application to update the corresponding PPT file when saving a modified presentation, can produce redundant data in a PPT file. In light of the fact, a novel steganographic method for PPT files based on file update is presented in this paper. The basic idea of the proposed method is first locating the redundant data derived from file updating, and then overwriting some fields of them with the encrypted secret message. If the space occupied by the existing redundant data within a file is not sufficient for embedding, we can employ the file update mechanism to adaptively augment the extra redundant data so as to satisfy the capacity requirement. Both the experimental results and analysis demonstrate that the presented method performs well in terms of imperceptibility, capacity, and security.