Long-form content is detailed and extensive, aiming to provide deep insights and establish expertise. Examples are blog posts, detailed articles and comprehensive guides. It's important to test this type of collateral to confirm you don't lose your audience, that it answers consumer questions and always good to have extra proofreading checks.
Questions you could ask:
"How informative do you feel this document is? Anything we could add?"
"Is the below copy engaging for you or do you lose focus because of a lack of interest? Is there anything you think we should take out because it wasn't helpful or relevant?"
"On a scale of 1-5, how clear is the point of this article? Do you have any questions after reading it?"