If you've downloaded Dropbox's desktop software to your Mac, you can navigate to the file in the Finder and control- or right-click on it. Select Share Dropbox link from the contextual menu. Nothing will seem to happen, but a link has been copied to your clipboard. Go to your email program, paste the link into the message, and send it out.
If you need to share a group of files and receive corrections and new versions, your best bet is to share a Dropbox folder. In the Finder, create a folder within your Dropbox folder, control-click on it, and select Share this folder. This opens your Web browser to a window where you can enter people's names and write a brief message describing the contents of the folder. Click Share folder to email instructions on how to view the folder.
Each person must sign up for a free Dropbox account, which means giving a name and email address to the service, and creating a password. Once they've done that, however, people can see and edit the folder's contents, add items, and download as they wish. Dropbox works with Windows, Mac, Linux, iOS, Android, BlackBerry, and Kindle Fire, so you can share big files with just about anyone.
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