I created a manual for a popular piece of equipment and placed it, as a PDF-file, on my website. The PDF contains a clickable link to the root domain of my website. Other websites have downloaded the PDF and put it on their own website, hosting it themselves. Since Google indexes PDFs, does the link in the PDF count as a regular 'link' from an external website, and does it have comparable value? Can't find any thoughts or studies about this anywhere.