The Url settings tab for a text element looks as below:

You can add a link in the PDF output for the text via the Url settings tab above. The actual link can be a constant like above, an Xpath expression, a variable, function or translation. Please note, that the url will not be highlighted as default (e.g. in blue with underscore), so if you want that you will need to set that up with the text style and Color style.