If we’re being honest, the best reason to avoid black hat tactics in SEO isn’t that they’re morally wrong1 but because they tend to get nuked pretty quickly. And when Google gives the ax to a particular tactic, they usually make a point of punishing websites that use them.
The most recent example of this involved a sneaky form of backlink building that took advantage of Google’s (at the time) lax approach to vetting PDFs. Although Google basically killed the effectiveness of link farms a while back, a similar tactic was discovered by Sophos, a company that offers Internet security services and tools.
Thanks to a new virus detection method, the team found hundreds of thousands of PDFs that were filled with keywords and backlinks, but the content was a random mishmash of gibberish. While this no longer flies with HTML files, PDFs were still inherently trusted by Google in the same way that .gov and .edu sites are assumed to be more reliable. As a result, although Sophos saw red flags, Google didn’t.
Once PDFs were filled with keyword soup and links, they were uploaded to authoritative websites that were likely hacked. To avoid people noticing that something hinky was going on, cloaking tactics were used.
It was necessary for Google to see these garbage documents without having regular people see them and wonder what was going on. This was accomplished through cloaking, where Google crawlers see a different version of a page than the normal user. When a website detects that a Google crawler is going through the site, a version of the page with the keywords and links is shown.
As you can see in the image below, this tactic was very effective for a number of binary trading broker services sites.
When people clicked on one of the PDF links, they were general taken to a website offering binary trading services.
Sophos stated that they reported the issue when they discovered it to Google, but the search engine did not respond. However, if you do a search for “binary trading austria” or “safe stock trading us,” both of which where previously littered with PDF results, you’ll see a normal selection of results.
This SEO tactic probably took a bit of time and effort, but Google probably killed it in less than a day.
 Especially since many used to be legitimate practices.