Usually we use Winzip to compress files but this formate is not acceptable to Gmail and may other email service provider due to doubt of virus infection , 7zip is software which not only compress at high ration but also acceptable by Gmail to upload.

7-Zip is a file archiver (compression) utility for Windows with a great array of features including:

• High compression ratio in new 7z format with LZMA compression
• Supported formats:
  • Packing / unpacking: 7z, ZIP, GZIP, BZIP2 and TAR
  • Unpacking only: ARJ, CAB, CHM, CPIO, DEB, DMG, HFS, ISO, LZH, LZMA, MSI, NSIS, RAR, RPM, UDF, WIM, XAR and Z.