First what is PageRank (PR)
Google PageRank is an important factor that determines the importance of a website or a blog in the eyes of the search engine, in other words the higher the PageRank the higher the webpage or a website will rank in the search engine!
To increase ranking the website needs to have lots of backlinks in the form of HyperLink and only backlinks that don't have rel="nofollow" are good for PageRank, so that is why it is so important that we don't allow any outgoing links to be Dofollow as our site will be used by those backlinks to increase their own Google PageRank.
The other reason why we should do it is because we need to keep search engine crawlers more on our site so that they will have enough time to properly index it, but if a crawler stumbles upon a link that doesn't have rel="nofollow" it will follow that link thus cutting short the indexing proses of our site.
So remember every time you create a DEMO page that is hosted on some other site or you add a download link again that is not part of your site make sure to add rel="nofollow" to all those links. The same applies to linking your social media profiles and fan pages like Facebook, Google Plus and Twitter.
How to add Nofollow Tag in HyperLinks
Here is a normal Dofollow Hyperlink (backlink)
This link will be followed and used in PageRank calculation!
Here is the same link but this time it will not be followed neither used for calculating PageRank!
<a rel="nofollow" href="URL">Download</a>
What links should be kept Dofollow
All the the internal links should always be kept Dofollow to allow crawlers freely end efficiently crawl your entire site.