answer:If the account is using the POP system for retrieving mail, there will be a setting allowing you to delete files on the server once they have been downloaded to a device. You need to turn this option off so files are available on both the web and on the iPhone. Depending on how the account is set up, this might work: in the ‘Mail, Contacts and Calendars’ section of the iPhone settings tap the Yahoo account, scroll down and tap ‘Advanced’, in the Advanced there is a setting called ‘Delete from Server’ which you want to set to ‘Never’.