In the past year I came across this interesting situation with a couple clients I'm managing Google Ads for; they were linking to PDF brochures on their website.

These brochures effectively served as an advertisement with phone # and email address listed on the bottom of each.

As a Google Ad Manager, I immediately found issue with this. Any Google Ad visitors that come to the website, click on the PDF brochure and decide to contact via the contact details on the PDF, can't be attributed back to the Google Ad Campaign.

Why does this matter?

Anyway I website visitor can become a lead/customer that you're not tracking as a conversion is what I call "leakage".  

The more leakage you have, the harder it is to optimize a PPC ad campaign to increase performance (lowering cost per conversion/increasing return on ad spend).

So I recommend not linking out to PDF brochures from your website. There's really no point, that is what your website is for. Just publish the info that is on the brochure, on your website.

It's a poor user experience making a website visitor download a PDF to get all of the required info they need to make an informed buying decision, and it's bad for conversions.