As an aside, there's no such thing as a 'fair use download.' It is illegal to download from anywhere other than an authorized retailer from whom you have purchased the book. Period.
Except if you pay a reprography fee in your country which means you can copy any book legally, and these reprography fees are forwarded to an US agency (along with fees from libraries) and then sent to WOTC so you paid for the permission to get a copy, and used an agency that is authorized to do so by law.
Also if we don't speak about the cases where you are actively encouraged to run demo games, but new players need access to run rulebooks, since that encouragement means that copying is part of normal use of the stuff, and it can place it in fair use.
Also if we speak about modules where you are expected and instructed to send some information to your players, but they made a module into a file you can't edit, so if you follow the instructions you would send the whole file.
So we have legit purchases.
We have copying that is legal in the country where they happen.
We have sends with a perfectly legit purchase, encouraged by copyright holder (if they encourage you to do something that also means they have to permit that)
We have people sharing the books in a party - which is common with hardcopies too, and a fair and common practice in the community.
We have people who bought the hardcopy and pirate the PDF since they say: "we already paid for the content"
And finally there are people who just take the copy.
Even the later can be:
Someone wanting to see it before buying, who wouldn't otherwise buy the book => results in an extra hardcopy sale
Someone who can't affort the book, but his party can, and if he can DM with the pirated copy the party will buy more books => extra sales
Someone who can afford the book, would buy the book, and buys it later => no real effect for WOTC
And a real lost sale
And the lost sale due to piracy vs legit pdf sales ratio are very different from this claimed ratio.