You know every iPhone, iPad and iPod using Safari does not have the ability to download files. The reason for this is that the iDevices themselves do not give the user access to a normal file system for file storage as you have on a Mac or Windows computer and so there is nowhere to "save" the downloaded file.
You should use server-side to upload their canvas as Base64 to write PNG files on your server, so they can save link, also it works with twitter and facebook.
If the user wants to save this screenshot, you give an instrucction how to take a screenshot while pressing both buttons, it automatically saves the screenshot on its local disk.