PDF pages are not uncommon on the Internet. They are used for various reasons, but site owners are not always able to properly optimize such files. But this can be done very easily. Experts recommend using regular html pages both for the site and for creating various books. However, PDF is often more convenient. Some webmasters believe that such pages cannot be optimized, therefore, the text that is in the PDF document does not work for promotion. But there really isn't much of a difference between html and PDF pages.
The PDF format is often used on websites, especially in the B2B field. Google, for example, started indexing them back in 2001. Yandex also indexes such files. Of course, if you are posting a PDF on your site, you want it to work in search. In the search results, in particular from Google, such files are even marked accordingly. A good and well-written title will help search engines better rank the file. Placing internal links to a PDF document on relevant pages will make it clear to Google that you want bots to index this file and subsequently rank.