With thanks to Peter Sauer, Apple expert, who tested this ´step by step´ guide on how to download a digital certificate onto an Apple device, and declared it of use to those that don´t find the option on the government website.
For iPad users, you need to use IOS Mail app and Safari to first mail the certificate to yourself and then download/import on iPad.
Update October 2019:the FNMT website now also has provisions for MAC users. Tip from one of our members: If you use a Mac you need to downgrade your Firefox browser which can be a pain. Best to keep both the old and new version. Once you have the certificate installed, you can update your browser to version 69.0.1
Member Julie Orsborn has managed to install a digital certificate on her iPad, with Safari browser, using the instructions in this article. You need to have downloaded it onto another device first, with passport protection, and then email a copy of the certificate to yourself, then follow instructions in the link.