One of the biggest advantages of owning an iPad Pro is that you can use Apple Pencil with it (which isn’t supported by other iPads). In iOS 11, Apple has added new features to enhance the functionality of the Apple Pencil. One such feature is Instant Markup.
Although this feature was first believed to allow iPad users to markup anything on the screen, the reality is that it works with PDFs and screenshots only.
If you want to markup a screen, tap the share icon, then choose Markup as PDF.