To copy files from your Mac to the tablet, you can drag them from the Finder into that File Sharing pane, then do a sync. To access those new files on your iPad, you open the app’s Document Manager, then tap on the Import button at the upper right. Keynote can import .keynote, .ppt, .pptx, .pps, and .ppsx files. Pages can do .pages, .docx, .doc, .dotx, and .dot formats. And Numbers can work with .numbers, .xls, .xlsx, .xlt, .xltx, .xlsm, .xla, .csv, and .txt.
Some document elements may be lost in translation. For example, if you import a Keynote presentation from your Mac to your iPad, it’ll lose any recorded or embedded audio, object groups, presenter notes, and 3D charts. If the iPad app doesn’t have one of the fonts used in the original, it’ll substitute it with one that matches it closely; if a match is not found, it’ll use Helvetica. When you first open the imported document, the app will show you a list of the changes.
When you want to copy a file from your iPad app to your Mac, you open up the app, select the document in Document Manager and hit the Export button. Then you can select the file-format in which you want to copy the file. All the iWork apps will, of course, export in their respective iWork ‘09 for Mac formats. They can also export to PDF; Pages can also save in the Microsoft Word (.doc) format.
The next time you sync, you select the app in the File Sharing section of the iTunes’ Apps tab, and either drag-and-drop files from there to your Mac or select the file and click the Save To button. You can also use that Export button to send iWork documents via Mail or to share them via iWork.com.