First finding from my DuckDuckGo search testing. That site: is not at all a reliable indicator of what is indexed, when using an extended URL. For instance, the PDFs of the Joint Nature Conservation Committee, UK…

site:http://jncc.defra.gov.uk/pdf/

One lone result in DuckDuckGo. However, search for…

“The Vascular Plant Red Data List for Great Britain”

And up it pops at…

http://jncc.defra.gov.uk/pdf/pub05_speciesstatusvpredlist3_web.pdf

So the PDFs at http://jncc.defra.gov.uk/pdf/ are in there then, but it seems they can only be surfaced in DuckDuckGo by using…

site:jncc.defra.gov.uk filetype:pdf