Pretty sure you don't open the PDF in a reader - in fact it looks like there is no "resulting PDF". Instead the PDF is hosted on the site, which you link to, and the analytics are run on the page.
Re:Show HN: Track and Control Your PDFs
by codingdave on Thursday June 07, 2018 @02:08PM
(#17258509)
At which point, why even bother with the PDF... just host your content on any web site, and run Google Analytics.
Re:Show HN: Track and Control Your PDFs
by kevin_b_er on Thursday June 07, 2018 @01:52PM
(#17258337)
So they're not PDFs anymore?
I haven't tried it, but it seems to me that this only works if the PDFs are stored on the site and viewed online, so essentially just regular analytics with an embedded document viewer.
Edit - in fact, the site badly needs “How it works” section, because otherwise it reads like yet another “self-destructing emails” type of product.