If you’re linking to something within the text of an article, you just make the relevant word(s) – called “anchor text” – into a link, and they’ll appear underlined. People know what that means, and that they’re free to click the link if they want to find out more. You don’t usually want them to, of course, but it’s a helpful part of web technology which avoids you having to repeat things, and makes an article more authoritative at the same time. However, a link at the end of an article – an exhortation to actually do something – is quite different. It’s a call to action, a link which you really do want people to click. So never treat it with the same subtlety. The two most common mistakes in a call to action are in the wording and the presentation. A label like “PDF Download” is not a call to action. If you want people to download that data sheet, then sell it! Firstly, tell them why they ought to click on the link: “See why the Blue Widget is so great in this datasheet” is much more like it. Secondly, make the link attractive. A big red button is a lot more exciting, and has been proven conclusively to work better in almost every situation. If you can show an image, illustrating the result of the click, even better.