Marit,
It depends on the version of Adobe you are using. In Acrobat 9 Pro there is a menu item to reduce file size. Go to Document / Reduce File Size (third section from the bottom). In earlier versions of Acrobat it think (I don't fully remember) it is under the File / Properties menu.
Another option is to post the document on your website and have your client download the file. (This way there is no email size restrictions.) To do this all you need to do is upload the file to your website, and place a link to this file somewhere on your website. (I have a "downloads" page on my website where I post files for clients.) After your client downloads the file you can remove the link so no one else downloads the file. (You can also create a separate download page for each client, and only give each specific client the link to their "download" page.)
Julie