
PDF documents are not just containers for text and graphics, PDFs are capable of containing whole files inside them -- making exchanging sets of documents much easier and more reliable. For example, if you're sending a PDF document for review, you could attach the source file (say a Word file) inside the PDF document. It's a good way to make sure files don't go missing. When the user double-clicks on the attachment icon, the file will open up the default viewer for that application.
The File Attachments window gives you a central place to view, insert, delete and export attachments, and you can also access all attachments from the Commenting panel.
Note: Recipients of your PDF documents containing attachments can only access them if they have an application that supports the format. For example, if you attach a Word file, the recipient must have Microsoft® Word installed.