If you right-click (control-click) the file's icon you get a contextual menu. One option is "Compress "<filename>" and that will create a ZIP file without any extra steps or operations.
However, the amount of compression you get depends on the file. Example: jpgs and pdfs are already compression formats and they may not shrink as much as, say, a Word file or PhotoShop file (.psd) will.
Test to see how much the above method compresses. If not enough, consider finding an e-mail servie that has more liberal attachment size limits or, if you need to routinely transfer a lot of big files, set up an ftp site. ftp (file transfer protocol) handles very large files.