You can't.
Just print it, sign it using pen and scan the resultant one back.
However, this is specific to family information form only. If you are applying online - you probably don't need to do it at all. In particular for other primary forms you can't do that because printed and scanned forms loose the verification bar codes at the end. Family information form has none so you don't have to worry about those plus you need to sign in several places on this one.