This is a common problem with Apple devices and there are two possible solutions.
The first is to install Dropbox - once that is installed you can long-press on the file you wish to download and select "Save to Dropbox", this will save the file to the Dropbox folder on your Apple device.
The second is to use an app called "iCab Mobile" - https://itunes.apple.com/us/app/icab-mobile-web-browser/id308111628?mt=8 - this is a feature-rich alternative to the Safari browser and will allow you to save files from our site direct to your iPad, iPod, or iPhone.
You can, of course, also download the files to your desktop or laptop computer and transfer them across using iTunes as usual.