DLsecure is just a toolbar that integrates into your browser and then it displays all sorts of ads, built-in search-results, word linking, etc. If you have a download manager, you can embed it into your browser (most download managers feature their own plugin). For this purpose I recommend on using Internet Download Manager or FlashGet. Both have the ability to integrate into your browser to catch the download links (PDF included). It's recommended to remove DLSecure from your computer to avoid any possible malware infection.