1st generation Apple Pencil compatibility

This support page details the iPad models with which the two models of Pencil are compatible:

Apple Pencil compatibility - Apple Support

Unlike third-party styli, that require specific App support, the Apple Pencil is supported at system-level. That said, many but not all Apps are optimised to use the Apple Pencil. You may wish to research any Apps in which you have interest to verify their level of integration with Pencil functionality.

To reiterate my earlier comment, different Apps offer different features.

Many native Apple Apps support “markup” - and will allow free-input from finger or pencil.

As a further example, Adobe Reader also supports free input and document signing of PDF documents. It may be worth noting, however, that a free-form signature applied to an electronic document may not have any legal standing in your region or legal jurisdiction.