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What is the difference between and implicit and an explicit hyperlink?

We often get asked why are web address like www.myhost.com does not work as an active link to the web address in a PDF. This posting explains the difference between a link that an application “guesses” should be treated as a web or mail link, and a link that includes associated information to tell the application that it should act in a particular way. Note that in this posting the entry www.myhost.com does not work as a link because I have not included an explicit link as part of this text, so the web browser thinks it is just pure text.

Explicit hyperlinks

Suppose you have a Word document, and you type the text “Click here to access my website”. This clearly will not go to your website when you click on it. However, if you select the text and use the menu option to Insert a Hyperlink, it will encode the text with an associated (hidden) string that tells Word to go to that URL when the item is clicked – and if you then export the Word document to a PDF it will ensure that the text string keeps this hidden association and the item will be shown as a link in the PDF and will work. So this is an explicitly coded link. If you have the URL as text in a document (e.g. in InDesign) it is the same – it is just text although many applications (e.g. almost all Adobe and Microsoft applications) may well ‘guess’ that you really mean the item to be a link – in fact it is difficult to stop them doing this if you don’t want them to. You can then tell InDesign to make it an explicit link as per Word or wait until it is output to PDF and use Adobe Acrobat, Tools option, Document processing (Create URLs) to make the links explicit, i.e. encoded into the underlying PDF file structure.

Implicit hyperlinks

If you open a PDF in Adobe Reader it will look at a text string that is probably a URL link and guess it should be a link and highlight it and act as if it was a real link – this is an implicit link, in that it implies its purpose from its structure. An implicit link will not be recognized and acted upon by Javelin and most other PDF readers – some PDF readers will work with implicit links and some will not. So, explicitly specifying text strings as links is the correct approach to take for safety.