To begin adding hyperlinks to your document, simply select the "Link" tool from the ribbon.

With the link tool selected, left-click and drag over the area of the PDF you wish to hyperlink. This will then prompt you with the hyperlink options.

In this prompt, you can either hyperlink to an external website or a specific page in your PDF. After confirming the hyperlink, a small border will appear around the hyperlinked area.
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You can adjust the visible properties of the hyperlink, such as the border colour, or the hyperlink location by using the select tool found on the ribbon.