Since Atticus exports your PDFs to be print ready, you'll find that URLs are not clickable within the PDF export. Only the Epub will be clickable.
Rather, a PDF file is flattened to be accepted by a printing company and ready to go for publishing.
However, there may be instances where you'll want to include URLs within your PDF as you may not be sending it off to print. If you want to override this, here's how you can make your Atticus PDF files include clickable links!
Step 1: Export your PDF file from Atticus and open it inside a PDF reader/editor like Adobe Acrobat
Step 2: Choose the option along the left to "Edit a PDF"
Step 3: Find the option to add "Links"
Step 4: Highlight the text you want to be a clickable link, and choose the following Link Settings:
a. Invisible Rectangle
b. Open a web page
Step 5: Click Next and type in your desired URL
Step 6: Save your new PDF
Step 7: Open to verify that the link is now present
And that's it! You can do that throughout your manuscript and ensure your PDF now has clickable URL links.