Packing a project will remove all the temporary (cache) files associated with a project. This includes things like thumbnail images and other items that the app can recreate from the project. Packing can reduce the amount of space a project uses on your device, and sometimes clears up any issues you might have with a specific project. Once a project is packed it may take longer to open the next time as the app has to recreate these temporary files. When a project is uploaded to Dropbox, these temporary files are not included in the upload which saves you bandwidth and space in your Dropbox.