I have tried recently Kiwix, a pretty nice application that allows to read Wikipedia offline. Kiwix is a free software, available for various systems including Android. Actually Kiwix reads ZIM files, which are special archive files designed to store wiki pages. The whole Wikipedia in English of February 2014 is stored in a unique 42 GB ZIM file (12 GB for the text only version), which can be downloaded from the kiwix website. Can we create a customary ZIM file containing a selection of Wikipedia articles? Yes, and this can be done simply by clicking on the “Create a Book” link in the Print/Export section of the left panel of a Wikipedia article. We may then add several articles to our book, and export it in various formats, including PDF, EPUB, and of course ZIM. Many books are already available in the Wikipedia Community Books Collection. There are alternatives to Kiwix, including for instance Xowa, which uses another file format similar to ZIM but which allows additionally to store Category/Portal structure.
Note. The ZIM file format uses the Lempel–Ziv–Markov chain compression algorithm (LZMA).