ZLib Inflate() failing with -3 Z_DATA_ERROR

Inflate() was failing because it was looking for GZip headers which were not present. If you initialize the stream with:

ret = inflateInit2(&strm, -MAX_WBITS);

Passing a negative window bits value prevents inflate from checking for gzip or zlib headers and unzipping works as expected.

That file that begins with 50 4B 03 04 is a zip file. The zlib library does not process zip files directly. zlib can help with the compression, decompression, and crc calculations. However you need other code to process the zip file format.

You can look at contrib/minizip in the zlib distribution, or libzip.