A link annotation can added just like any other annotation, but opening the annotation toolbar and selecting the annotation tool. Here's how it looks:
So what can you do with a link annotation?
A link annotation can be added anywhere in the PDF, be it text, an image, or just some area on the PDF. It's two main purposes are to link to another page in the same PDF or to link to a webpage. When you've finished creating your link annotation then it will work just as expected, once you tap on the text, image or area where you've added your link annotation it will direct you to the page or webpage.
Link to PDF Page:
Link to a Webpage:
Link-annotation creation and editing is a Pro feature and requires a valid PDF Viewer Pro subscription.