In iOS 11 you can easily add annotations and other markup elements to documents.
To do so, open up a document on your device – perhaps an emailed PDF, or a document you have lying around in iCloud.
Then tap the markup icon in the top right corner, use your finger to make quick notes or add pointers, then tap Done when you’ve finished. The annotations will be safely saved to the document. You can then share it by email or any other method via the Share button.