If you're planning to have iOS load the certificate from a web page, you'll just need to ensure the web server provides the correct MIME type for it. For a
pem formatted certificate, the MIME type is
application/x-x509-ca-cert. If the MIME type is correct, iOS can accept a PEM-formatted certificate as-is, present some "are you sure to accept..." prompts and install it as a minimal "configuration profile" with just the certificate in it.
If you need to feed iOS both the certificate and the corresponding private key (i.e. you'd need the certificate not for verifying the identity of some service, but to actually use the certificate for authenticating into something), then you might want to package both the certificate and the key into a single PKCS#12 file (suffix
.p12), and serve it up as MIME type
application/x-pkcs12. The conversion to PKCS#12 format can be done with the
openssl pkcs12 -export command: see
man pkcs12 for details.
Another option would be to wrap the certificate into an Apple-style "configuration profile" with a
.mobileconfig suffix (and the corresponding MIME type
application/x-apple-aspen-config). However, creating a
.mobileconfig file is tricky unless you have some tool that can do it for you. The
.mobileconfig file is an Apple
plist, i.e. a XML file with a specific structure, and it can optionally be signed.