Signing and marking up pdf files is very easy in iOS. If you open pdf:s in iOS Mail, Notes, Messages, Safari, Files, Photos, and iBooks, you can use the Markup tool to sign or draw on them. Just tap the markup icon and choose tool to use.
If you have an email, webpage or something else you want to markup, you can easily convert it to pdf first. Then save the resulting pdf in Files or some other app with Markup support.