In general, files generated by the same program (Microsoft Word, for example) should be shareable cross-platform, though on the machine with the newer version of the software, you MAY have to “save down”, or save in a version that’s compatible with the older software. Where you might end up with a problem is in attaching files in your email client — sometimes Windows doesn’t like files attached on a Mac system, and vice versa, but if you’re having that problem you can often get around it simply by placing all of the files you want to send back and forth into an archive format that can be read by either platform, like the universal Zip format. What you may want to consider instead, especially if the files are going to be large, is using an online file-uploading service like YouSendIt.com, and simply sending the files to yourself. That way you don’t have to worry about whether the attachments will be readable at the other end, and you’ll avoid running into problems caused by servers that can only handle smaller-sized attachments.