Your question about SEO and gated content is likely one of two queries: How do I keep the gated content off search engine result pages or how do I allow search engines to index my gated content?
Answer: You have the power to make Google, Bing and others index any URL (PDF’s have URLs) you want.
You also have the power to block Google, Bing and others from indexing any URL you want.
This will automatically make the content available via search. Simply add a meta no-index no-follow tag to the HTML of that content. This will block the content from appearing in search results.
More details about SEO and gated content:
I want to stress that there is no ambiguity in my advice as a search marketing consultant on this issue, and if Googling around and reading other articles has caused any confusion on what to do, don’t bother referencing those articles.
This is a very straight forward issue – you have the power to have Google index content on your site, or to not index content on your site.
Example of gated content being indexed:
HubSpot, a SEO and inbound marketing industry leader, lets you access a PDF of their marketing guide via search. Originally, it was offered on a landing page that required you to fill out a lengthy form.
Result #6 for me is the PDF. It may be in a different position for you but it should be on there.
I was at an agency 3 years ago and we had one of our Ebooks show up on Google. At the time, no one knew how to solve this problem.
Crawlers can go link to link to link across your website, so if there is any place that happens to link to the PDF the crawlers will find it. This includes external websites linking to the gated content.
For example, I download your PDF and then link to it in a new blog on my own website. The crawlers will find it on my website and then the URL/PDF will now be in search results.
This is why when you really want to GATE the content, you add a no-index, no follow tag or use the remove URL function in Webmaster Tools.
Conclusion regarding SEO and gated content:
If you’re interested in search optimizing the PDF, here’s some info on that.
I actually wrote about this exact topic in April – http://seoshrugged.com/2014/04/13/de-indexing-pdfs-and-gated-content/
And here’s Google’s take on the subject – http://googlewebmastercentral.blogspot.com/2011/09/pdfs-in-google-search-results.html.