Hi,
You can use our Boxcryptor plugin for the Google Chrome browser which can decrypt an encrypted file directly in the browser while downloading it. Other than that, it is not possible to decrypt a file without our Boxcryptor software.
You can find the plugin (which is a beta version) in the Chrome Web Store: https://chrome.google.com/webstore/detail/boxcryptor/ijmndaodmdjamfepoijpolhjddgfgmme?hl=de
Best regards,
Robert