Yep, check the HTTP headers with webbug or there are a bunch of browser plugins that will let you see the headers for the document.
That said, I would push to drive the links to the page though rather than the document itself and just create a nice page that houses the document and make that the link target.
You could even make the PDF link only available by email once they have singed up or some such as canonical is only a directive and you would still be better getting those links flooding into a real page on the site.
You could even offer up some HTML to make this easier for folks to link to that linked to your main page. If you take a look at any savvy infographics etc folks will try to draw a link into a page rather than the image itself for the very same reasons.
If you look at something like the Noobs Guide to Online Marketing from Unbounce then you will see something like this as the suggested linking code:
[](<strong>http://unbounce.com/noob-guide-to-online-marketing-infographic/</strong>)
[
](<strong>http://unbounce.com/noob-guide-to-online-marketing-infographic/</strong>)
[](<strong>http://unbounce.com/noob-guide-to-online-marketing-infographic/</strong>)
Unbounce – The DIY Landing Page Platform
So, the image is there but the link they are pimping is a standard page:
http://unbounce.com/noob-guide-to-online-marketing-infographic/
They also cheekily add an extra homepage link in as well with some keywords and the brand so if folks don't remove that they still get that benefit.
Ultimately, it means that when links flood into the site they benefit the whole site rather than just promote one PDF.
Just my tuppence!
Marcus