Wowio.com used to do that. They would create PDF files with a couple of ads at the beginning and end of the file, and insert your name into it as well. They'd let you download for free something like 3 books a day, up to 20 a month. Unfortunately, they couldn't get enough income from that since they seemed to only have less than a dozen sponsors. They discontinued the business plan several months before I finished downloading the ones I wanted.
In the late 60s or early 70s, I used to get paperback books with an advertising insert in the middle. A lot of them had either cigarette advertising or book club mailers which I usually ripped out.
I wouldn't buy a book with animated ads or lots of ads interspersed within the text, but I didn't find the single page ads at the beginning and end of the PDFs to be egregiously annoying. But I think that it would need to be at a substantial discount (free is good) for me to put up with any ads.