There are two ways in which a Lean proof can be written - a proof term (without the by keyword) and a sequence of tactics (with the by keyword).
Many theorems in Mathlib are written as proof terms. When you try to check them in Paperproof, you get a "Not within theorem" notification.

At the moment, Paperproof cannot render proof term proofs - Paperproof only renders parts of the proof that start with by, meaning some theorems from Mathlib, like the one above, are not even detected as theorems by Paperproof.
Any such theorem can be rewritten into a sequence of fake applys and intros, so that it looks something like this:
theorem norm_image_sub_le_of_norm_deriv_le {C : ℝ} (hf : ∀ x ∈ s, DifferentiableAt 𝕜 f x) (bound : ∀ x ∈ s, ‖deriv f x‖ ≤ C) (hs : Convex ℝ s) (xs : x ∈ s) (ys : y ∈ s)
: ‖f y - f x‖ ≤ C * ‖y - x‖ := by
apply hs.norm_image_sub_le_of_norm_hasDerivWithin_le
intros x hx
let smth := hf x hx
apply DifferentiableAt.hasDerivAt at smth
apply HasDerivAt.hasDerivWithinAt (f' := deriv f x) at smth
exact smth
exact bound
exact xs
exact ysWhich then can be rendered as follows:

Which immediately makes the theorem much more readable.