Should I put a hyphen between off and line, or can I just use the word offline, so my document is consistent?

Be consistent. Whoever ultimately publishes it will do the same. When compound words are first coined, they are usually hyphenated, but often the hyphen is dropped as they become more widely used. Offline is headed towards losing the hyphen, but many people still use it. One simple test is to see whether your spell checker flags it.