wmf files are often stored compressed compressed with lz compression in word files They are normally stored in a compressed format using the LZ compression algorithm in the format used by GNU Zip deflate/inflate with a 32k window. The format is zlib format . The only metafile compression version number currently defined identifies this format and is analogous to the PNG compression type value in the PNG file format. Peter Brandstrom - D/Z took at look at it and said The compressed data begins with a zlib header which seems to be two bytes containing the compression method, window size and check byte. The zlib default is a 32k window.