Traditionally, Google collects content from websites to display selected snippets. But now we see the paragraphs under the headings in the search results, selected by Google from the cumbersome PDF documents. And the fact that we see this indicates that sometimes Google PDF is preferable to web pages.
What does this mean for SEO?
The most important conclusion for SEO is that PDF files can now receive snippets, also called "zero position".
This means that best practices that apply to optimizing web content for selected fragments can now be applied to PDF files.
One of the trends in SEO in 2019 will be the popularity among users of the so-called “zero position” in search results. This is when the answer to the question is in the snippet on the first page, which, on the one hand, reduces the number of visits to the site, but on the other hand meets the needs of users, which means Google will be pleased with you.